diff --git a/js/src/nanojit/NativeX64.cpp b/js/src/nanojit/NativeX64.cpp
index a08b0b0a151b..4974d1c9925f 100644
--- a/js/src/nanojit/NativeX64.cpp
+++ b/js/src/nanojit/NativeX64.cpp
@@ -171,9 +171,9 @@ namespace nanojit
     static inline uint64_t mod_disp32(uint64_t op, Register r, Register b, int32_t d) {
         NanoAssert(IsGpReg(r) && IsGpReg(b));
         NanoAssert((b & 7) != 4); // using RSP or R12 as base requires SIB
-        if (isS8(d)) {
+        uint64_t mod = (((op>>24)&255)>>6); // mod bits in addressing mode: 0,1,2, or 3
+        if (mod == 2 && isS8(d)) {
             // op is:  0x[disp32=0][mod=2:r:b][op][rex][len]
-            NanoAssert((((op>>24)&255)>>6) == 2); // disp32 mode
             int len = oplen(op);
             op = (op & ~0xff000000LL) | (0x40 | (r&7)<<3 | (b&7))<<24; // replace mod
             return op<<24 | int64_t(d)<<56 | (len-3); // shrink disp, add disp8