diff --git a/llvm/test/TableGen/GlobalISelEmitter-zero-instr.td b/llvm/test/TableGen/GlobalISelEmitter-zero-instr.td
new file mode 100644
--- /dev/null
+++ b/llvm/test/TableGen/GlobalISelEmitter-zero-instr.td
@@ -0,0 +1,8 @@
+// RUN: llvm-tblgen -gen-global-isel -optimize-match-table=false -I %p/../../include -I %p/Common %s -o /dev/null --warn-on-skipped-patterns 2>&1 < %s 2>&1 | FileCheck %s
+
+include "llvm/Target/Target.td"
+include "GlobalISelEmitterCommon.td"
+
+// CHECK: warning: Skipped pattern: Dst pattern root isn't a known leaf
+def : Pat<(zext (i16 (trunc i32:$src))),
+          (i32 $src)>;
diff --git a/llvm/utils/TableGen/GlobalISelEmitter.cpp b/llvm/utils/TableGen/GlobalISelEmitter.cpp
--- a/llvm/utils/TableGen/GlobalISelEmitter.cpp
+++ b/llvm/utils/TableGen/GlobalISelEmitter.cpp
@@ -5086,9 +5086,9 @@
 
   if (Dst->isLeaf()) {
     Record *RCDef = getInitValueAsRegClass(Dst->getLeafValue());
-
-    const CodeGenRegisterClass &RC = Target.getRegisterClass(RCDef);
     if (RCDef) {
+      const CodeGenRegisterClass &RC = Target.getRegisterClass(RCDef);
+
       // We need to replace the def and all its uses with the specified
       // operand. However, we must also insert COPY's wherever needed.
       // For now, emit a copy and let the register allocator clean up.