diff --git a/src/IceTargetLoweringX86BaseImpl.h b/src/IceTargetLoweringX86BaseImpl.h
index 38dfba7..941074b 100644
--- a/src/IceTargetLoweringX86BaseImpl.h
+++ b/src/IceTargetLoweringX86BaseImpl.h
@@ -3216,6 +3216,8 @@
         // Bitcast requires equal type sizes, which isn't strictly the case
         // between scalars and vectors, but to emulate v4i8 vectors one has to
         // use v16i8 vectors.
+        assert(getFlags().getApplicationBinaryInterface() != ABI_PNaCl &&
+               "PNaCl only supports real 128-bit vectors");
         _movd(Dest, legalize(Src0, Legal_Reg | Legal_Mem));
       } else {
         _movp(Dest, legalizeToReg(Src0));