diff --git a/content/canvas/src/WebGLContextGL.cpp b/content/canvas/src/WebGLContextGL.cpp
index 6c4fd58acfbb..8b37fd659eca 100644
--- a/content/canvas/src/WebGLContextGL.cpp
+++ b/content/canvas/src/WebGLContextGL.cpp
@@ -3626,6 +3626,9 @@ WebGLContext::ShaderSource(nsIWebGLShader *sobj, const nsAString& source)
     if (!GetConcreteObjectAndGLName("shaderSource: shader", sobj, &shader, &shadername))
         return NS_OK;
 
+    if (!NS_IsAscii(nsPromiseFlatString(source).get()))
+        return ErrorInvalidValue("shaderSource: non-ascii characters found in source");
+
     shader->SetSource(NS_LossyConvertUTF16toASCII(source));
 
     shader->SetNeedsTranslation();