diff --git a/src/com/google/javascript/jscomp/deps/JsFileParser.java b/src/com/google/javascript/jscomp/deps/JsFileParser.java
index 654747ee47f..d53a7f1182c 100644
--- a/src/com/google/javascript/jscomp/deps/JsFileParser.java
+++ b/src/com/google/javascript/jscomp/deps/JsFileParser.java
@@ -199,9 +199,8 @@ private DependencyInfo parseReader(String filePath,
 
   private void setModuleType(ModuleType type) {
     if (moduleType != type && moduleType != ModuleType.NON_MODULE) {
-      // TODO(sdh): should this be an error?
       errorManager.report(
-          CheckLevel.WARNING, JSError.make(ModuleLoader.MODULE_CONFLICT, file.toString()));
+          CheckLevel.ERROR, JSError.make(ModuleLoader.MODULE_CONFLICT, file.toString()));
     }
     moduleType = type;
   }
diff --git a/src/com/google/javascript/jscomp/deps/ModuleLoader.java b/src/com/google/javascript/jscomp/deps/ModuleLoader.java
index 37be3bbda39..0be407b1f07 100644
--- a/src/com/google/javascript/jscomp/deps/ModuleLoader.java
+++ b/src/com/google/javascript/jscomp/deps/ModuleLoader.java
@@ -39,7 +39,7 @@
  */
 public final class ModuleLoader {
 
-  public static final DiagnosticType MODULE_CONFLICT = DiagnosticType.warning(
+  public static final DiagnosticType MODULE_CONFLICT = DiagnosticType.error(
       "JSC_MODULE_CONFLICT", "File has both goog.module and ES6 modules: {0}");
 
   /** According to the spec, the forward slash should be the delimiter on all platforms. */
diff --git a/test/com/google/javascript/jscomp/LazyParsedDependencyInfoTest.java b/test/com/google/javascript/jscomp/LazyParsedDependencyInfoTest.java
index a585ebfcb16..e609e18987a 100644
--- a/test/com/google/javascript/jscomp/LazyParsedDependencyInfoTest.java
+++ b/test/com/google/javascript/jscomp/LazyParsedDependencyInfoTest.java
@@ -20,6 +20,7 @@
 
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
+import com.google.javascript.jscomp.CompilerOptions.LanguageMode;
 import com.google.javascript.jscomp.deps.DependencyInfo;
 import com.google.javascript.jscomp.deps.ModuleLoader;
 import com.google.javascript.jscomp.deps.SimpleDependencyInfo;
@@ -122,15 +123,17 @@ public void testParseIsLazy() {
 
   public void testModuleConflict() {
     Compiler compiler = new Compiler();
-    compiler.initOptions(new CompilerOptions());
+    CompilerOptions options = new CompilerOptions();
+    options.setLanguageIn(LanguageMode.ECMASCRIPT6_STRICT);
+    compiler.initOptions(options);
     JsAst ast = new JsAst(SourceFile.fromCode("file.js", "export let foo = 42;"));
     SimpleDependencyInfo delegate =
         new SimpleDependencyInfo("", "my/js.js", EMPTY, EMPTY, ImmutableMap.of("module", "goog"));
     DependencyInfo info = new LazyParsedDependencyInfo(delegate, ast, compiler);
 
-    assertThat(Arrays.asList(compiler.getErrorManager().getWarnings())).isEmpty();
+    assertThat(Arrays.asList(compiler.getErrorManager().getErrors())).isEmpty();
     assertThat(info.getLoadFlags()).containsExactly("module", "es6", "lang", "es6");
-    assertThat(Arrays.asList(compiler.getErrorManager().getWarnings()))
+    assertThat(Arrays.asList(compiler.getErrorManager().getErrors()))
         .containsExactly(JSError.make(ModuleLoader.MODULE_CONFLICT, "my/js.js"));
   }