diff --git a/sys/sparc64/sparc64/interrupt.S b/sys/sparc64/sparc64/interrupt.S
index 75a97275301..b71a05da98d 100644
--- a/sys/sparc64/sparc64/interrupt.S
+++ b/sys/sparc64/sparc64/interrupt.S
@@ -83,8 +83,11 @@ ENTRY(intr_vector)
 	 * The 2nd word points to code to execute and the 3rd is an argument
 	 * to pass.  Jump to it.
 	 */
-	brnz,a,pt %g3, 1f
-	 nop
+	brnz,pt %g3, 1f
+	/*
+	 * NB: Zeus CPUs set some undocumented bits in the first data word.
+	 */
+	 and	%g3, IV_MAX - 1, %g3
 	jmpl	%g4, %g0
 	 nop
 	/* NOTREACHED */