diff --git a/mozilla/content/html/style/src/nsDOMCSSDeclaration.cpp b/mozilla/content/html/style/src/nsDOMCSSDeclaration.cpp
index 9e5dc701366..88dc30ca84f 100644
--- a/mozilla/content/html/style/src/nsDOMCSSDeclaration.cpp
+++ b/mozilla/content/html/style/src/nsDOMCSSDeclaration.cpp
@@ -198,6 +198,9 @@ nsDOMCSSDeclaration::RemoveProperty(const nsAString& aPropertyName,
   }
 
   nsCSSProperty prop = nsCSSProps::LookupProperty(aPropertyName);
+  if (prop == eCSSProperty_UNKNOWN) {
+    return NS_OK;
+  }
 
   decl->GetValue(prop, aReturn);
 
diff --git a/mozilla/layout/style/nsDOMCSSDeclaration.cpp b/mozilla/layout/style/nsDOMCSSDeclaration.cpp
index 9e5dc701366..88dc30ca84f 100644
--- a/mozilla/layout/style/nsDOMCSSDeclaration.cpp
+++ b/mozilla/layout/style/nsDOMCSSDeclaration.cpp
@@ -198,6 +198,9 @@ nsDOMCSSDeclaration::RemoveProperty(const nsAString& aPropertyName,
   }
 
   nsCSSProperty prop = nsCSSProps::LookupProperty(aPropertyName);
+  if (prop == eCSSProperty_UNKNOWN) {
+    return NS_OK;
+  }
 
   decl->GetValue(prop, aReturn);