--- a/src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.asm.amd64/src/org/graalvm/compiler/asm/amd64/AMD64BaseAssembler.java	Thu Nov 14 11:16:14 2019 -0800
+++ b/src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.asm.amd64/src/org/graalvm/compiler/asm/amd64/AMD64BaseAssembler.java	Thu Nov 14 12:21:00 2019 -0800
@@ -944,7 +944,7 @@
     }
 
     public final boolean vexPrefix(Register dst, Register nds, Register src, AVXSize size, int pp, int mmmmm, int w, int wEvex, boolean checkAVX) {
-        if (isAVX512Register(dst) || isAVX512Register(nds) || isAVX512Register(src)) {
+        if (isAVX512Register(dst) || isAVX512Register(nds) || isAVX512Register(src) || size == AVXSize.ZMM) {
             evexPrefix(dst, Register.None, nds, src, size, pp, mmmmm, wEvex, Z0, B0);
             return true;
         }
@@ -953,7 +953,7 @@
     }
 
     public final boolean vexPrefix(Register dst, Register nds, AMD64Address src, AVXSize size, int pp, int mmmmm, int w, int wEvex, boolean checkAVX) {
-        if (isAVX512Register(dst) || isAVX512Register(nds)) {
+        if (isAVX512Register(dst) || isAVX512Register(nds) || size == AVXSize.ZMM) {
             evexPrefix(dst, Register.None, nds, src, size, pp, mmmmm, wEvex, Z0, B0);
             return true;
         }