diff --git a/src/main/java/com/google/devtools/build/lib/syntax/Environment.java b/src/main/java/com/google/devtools/build/lib/syntax/Environment.java
index 90e20da..3d09253 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/Environment.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/Environment.java
@@ -692,7 +692,7 @@
       Frame globalFrame = new Frame(mutability, parent);
       Frame dynamicFrame = new Frame(mutability, null);
       if (semantics == null) {
-        semantics = SkylarkSemantics.DEFAULT_SEMANTICS;
+        throw new IllegalArgumentException("must call either setSemantics or useDefaultSemantics");
       }
       if (importedExtensions == null) {
         importedExtensions = ImmutableMap.of();