1 files changed, 5 insertions, 0 deletions
diff --git a/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java b/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
index e0410dbb4d..b91e57f14b 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
@@ -205,6 +205,11 @@ public class ProtoCompileActionBuilder {
                 }
                 return value.toString();
               }
+
+              @Override
+              public String lookupFunction(String name, String param) throws ExpansionException {
+                throw new ExpansionException(String.format("$(%s) not defined", name));
+              }
             });
       } catch (ExpansionException e) {
         // Squeelch. We don't throw this exception in the lookupMakeVariable implementation above,