diff --git a/src/main/java/build/buf/protovalidate/CompiledProgram.java b/src/main/java/build/buf/protovalidate/CompiledProgram.java
index 8f39e82d..e41e8fde 100644
--- a/src/main/java/build/buf/protovalidate/CompiledProgram.java
+++ b/src/main/java/build/buf/protovalidate/CompiledProgram.java
@@ -105,8 +105,12 @@ final class CompiledProgram {
       if (Boolean.TRUE.equals(value)) {
         return null;
       }
+      String message = this.source.message;
+      if (message.isEmpty()) {
+        message = String.format("\"%s\" returned false", this.source.expression);
+      }
       RuleViolation.Builder builder =
-          RuleViolation.newBuilder().setRuleId(this.source.id).setMessage(this.source.message);
+          RuleViolation.newBuilder().setRuleId(this.source.id).setMessage(message);
       if (rulePath != null) {
         builder.addAllRulePathElements(rulePath.getElementsList());
       }