OPINION

Robert Cutler appeals from his conviction of conspiracy to commit mail fraud and arson in violation of 18 U.S.C. § 371 , Count One, use of an explosive to destroy a building in violation of 18 U.S.C. § 844 (i), Count Two, and mail fraud in violation of 18 U.S.C. § 1341 , Counts Three through Six. We affirm the conspiracy and mail fraud charge and reverse the conviction for the use of an explosive to destroy a building.

I. OPERATIVE FACTS

On December 11, 1976, Cutler’s warehouse was destroyed by a fire. Approximately twenty gallons of gasoline were spread throughout the warehouse and out the door into the alley. The “trailer” of gasoline that was spread from the ware-, house into the alley was ignited by a match. At trial, counsel entered into a written stipulation that the building was destroyed by an “explosion” within the meaning of 18 U.S.C. § 844 (i).

The evidence adduced at trial indicated that Cutler paid his employee, Walter Levoff to hire an arsonist to burn the warehouse. During Levoff’s testimony, the defense attempted to cross-examine him concerning other fires that had occurred on the premises of other businesses in which he had been involved. The government objected to any further questioning on this point. The court required the defense to defer questioning of Levoff on this subject until after Cutler’s testimony. The court also ruled that no extrinsic evidence would be received on this point. After Cutler testified, the defense called Levoff to the stand and examined him about his participation in approximately six fires dating back to 1969. Levoff denied participating in the setting of any of these fires.

II. ISSUES ON APPEAL

Cutler seeks reversal on the following grounds:

One. The conviction for the use of an explosive was based on a written stipulation entered into upon a mutual mistake of law.

Two. The court abused its discretion in restricting cross-examination of a key government witness.

Three. The court improperly precluded the introduction of extrinsic evidence of other fires of an incendiary origin.

*1248 III. EFFECT OF STIPULATION ENTERED INTO UNDER A MISTAKE OF LAW

At trial, defense counsel entered into a written stipulation that the means used to destroy Cutler’s warehouse constituted an “explosive” within the meaning of § 844(i). 1 After Cutler’s conviction this court held, in United States v. Gere, 662 F.2d 1291 (9th Cir. 1981), that a fire set for delayed ignition by the use of trailers was not an explosive within the meaning of § 844(i). Id. at 1295-96. Thus, the written stipulation entered into during the trial of this matter was based on an erroneous understanding of the law as interpreted by this court in Gere .

Cutler argues that it would be “unconscionable” to affirm his conviction on the “explosives” charge where it appears that his attorney was in error as to the legal effect of the facts upon which the stipulation was premised. We agree. It is our view that it would be manifest injustice to permit a conviction to stand where counsel have entered into a stipulation that certain facts established an element of the offense when, as a matter of law, assuming the truth of such facts, no crime was proved. See Marshall v. Emersons, Ltd., 593 F.2d 565, 568 (4th Cir. 1979). If we were to affirm this charge, Cutler would stand convicted of an offense which under Gere he did not commit.

Nothing we say here should be construed as a holding that defense counsel may escape the consequences of a stipulation solely because it was entered into without a full understanding of the law. We hold, instead, that upon an appeal from a criminal charge, an appellant may be relieved of a stipulation, when it would be manifestly unjust to uphold a conviction where, because of an intervening change in the law, there is insufficient evidence to establish the commission of a crime.

In the matter before us, without the stipulation, there is insufficient evidence to establish that an explosive was used as required under § 844(i). Accordingly, Cutler’s conviction must be reversed on this charge.

IV. LIMITATION ON CROSS-EXAMINATION

Cutler seeks reversal based on the limitation of his cross-examination of Levoff concerning other fires that had occurred at buildings in the area. The sixth amendment guarantees defendant’s right to cross-examine witnesses to challenge their credibility. Davis v. Alaska, 415 U.S. 308, 315-16 , 94 S.Ct. 1105, 1109-10 , 39 L.Ed.2d 347 (1974). The scope of the cross-examination of a witness is subject to the discretion of the trial court and is reviewable solely for an abuse of that discretion. United States v. Bleckner, 601 F.2d 382, 385 (9th Cir. 1979). When the government’s case turns on the credibility of a witness, defense counsel must be given the maximum opportunity to cross-examine and test credibility. Burr v. Sullivan, 618 F.2d 583, 587 (9th Cir. 1980).

We find that the trial court committed no error as to the cross-examination of Levoff. The effect of the court’s ruling limiting Levoff’s cross-examination was simply to delay, not restrict, a testing of his credibility; defense counsel had a maximum opportunity to test Levoff’s credibility. Ultimately Levoff was questioned as to the earlier fires upon which Cutler bases his claim of improper restriction of Levoff’s testimony. In addition Levoff was questioned as to his plea agreement, his prior felony conviction, his false testimony in an *1249 unrelated proceeding, his prior parole violation, and his motive to testify against Cutler. This impeaching evidence gave the jury an adequate opportunity to make a discriminating appraisal of the bias and motive of the witness. See Bleckner, 601 F.2d at 385 (test of abuse of discretion by trial court is whether the jury had' sufficient information to appraise the biases and motives of the witness); Skinner v. Cardwell, 564 F.2d 1381, 1389 (9th Cir. 1977), cert. denied, 435 U.S. 1009 , 98 S.Ct. 1883 , 56 L.Ed.2d 392 (1978) (no denial of effective cross-examination for impeachment where the jury has sufficient information to appraise the bias and motives of the witness).

Cutler’s contention that Levoff’s testimony concerning previous fires should have been admitted under Fed.R.Evid. 404(b) must also fail. Rule 404(b) permits evidence of specific acts as “proof of motive, opportunity, intent, preparation, plan, knowledge, identity, or absence of mistake or accident.” The trial judge, however, has the discretion to exclude such evidence and his decision will be overturned on appeal only if it has been abused. See United States v. Green, 648 F.2d 587, 592-93 (9th Cir. 1981). As discussed above, the trial court did not abuse its discretion; Levoff ultimately testified as to the previous fires.

V. EXCLUSION OF EXTRINSIC EVIDENCE

Cutler also challenges the court’s ruling precluding extrinsic evidence concerning Levoff’s alleged history of committing arson. Extrinsic evidence of a prior inconsistent statement is admissible under Fed.R.Evid. 613(b). Rule 613(b) requires that the witness must be “afforded an opportunity to explain or deny” the prior inconsistent statement and the opposite party must be “afforded an opportunity to interrogate him thereon.. . . ” Cutler failed to lay the required foundation and the evidence was therefore properly excluded under Rule 613(b). See United States v. Williams, 668 F.2d 1064 , 1068-69 & n.9 (9th Cir. 1981). No error was committed by the trial judge under Rule 613(b).

Cutler does not dispute that the Federal Rules of Evidence do not permit extrinsic evidence of specific acts of a witness’ conduct to attack his credibility. Fed. R.Evid. 608(b); see United States v. Wood, 550 F.2d 435, 441 (9th Cir. 1976). Instead he argues that extrinsic evidence of Levoff’s alleged involvement in prior instances of arson should properly have been admitted under Fed.R.Evid. 404(b) (evidence of specific acts admissible to prove, inter alia, motive, intent, or plan). Cutler’s statement of the law is correct. See, e.g., United States v. Batts, 573 F.2d 599, 602-03 (9th Cir. 1978) (vacating 558 F.2d 513 (9th Cir. 1977). Where, however, a trial judge has excluded evidence, “[ejrror may not be predicated upon [such] a ruling . . . unless a substantial right of the party is affected, and . . . the substance of the evidence was made known to the court by offer or was apparent from the context within which questions were asked.” Fed.R.Evid. 103(a), 103(a)(2). Cutler contends that the following extrinsic evidence should have been admitted: (1) a witness’ testimony about Levoff’s prior arson activities; (2) a government report; and (3) evidence of other fires.

First, the entire offer of proof concerning a witness who would testify to Levoff’s prior modus operandi is contained in the following colloquy:

Cutler did not identify any witness nor state the anticipated substance of his testimony.

*1250 Second, Cutler argues that the trial judge refused to permit the introduction of extrinsic evidence of a government report. In his opening brief he argues:

Brief for Appellant at 18.

The entire record concerning the report is as follows:

RT: at 292-93.

There was no response to Ms. Stock’s statement. Cutler’s attorney failed to pursue this inquiry and did not specify which document he had in mind, or explain the materiality or admissibility thereof.

Third, Cutler maintains an offer of proof was made that eight buildings at which Levoff had worked burned down. He states that “Again, the defense offer of proof was that four buildings designed by Levoff, plus four others that Levoff had already denied working at all burned down or had fires.” (RT: 500). The cited portion of the record relates to the proper scope of cross-examination of Levoff. 2

Cutler failed to make known to the court any records, documents, or witnesses to substantiate his “offers of proof” that Levoff had burned down other buildings and extorted money from the owners. His offers of proof were inadequate.

VI. CONCLUSION

The judgment of conviction as to Count Two is reversed and remanded to the district court for a new trial in the event that the government determines that it has facts in its possession concerning the use of an explosive which distinguish this case from Gere . The judgment as to the remaining counts is affirmed.

. The written stipulation in question, which pertained only to Count Two, provided as follows:

. As previously discussed, the Court later permitted Levoff to be questioned about the fires. See § IV supra.