diff --git a/Lib/ufo2ft/_compilers/baseCompiler.py b/Lib/ufo2ft/_compilers/baseCompiler.py
index d1b6c1fd..2e877a63 100644
--- a/Lib/ufo2ft/_compilers/baseCompiler.py
+++ b/Lib/ufo2ft/_compilers/baseCompiler.py
@@ -326,8 +326,8 @@ def _compileNeededSources(self, designSpaceDoc):
 
         # If the feature files are compatible between the sources, we can save
         # time by building a variable feature file right at the end.
-        can_optimize_features = self.variableFeatures and _featuresCompatible(
-            designSpaceDoc
+        can_optimize_features = self.variableFeatures and all(
+            _featuresCompatible(doc) for doc in interpolableSubDocs
         )
         if can_optimize_features:
             self.logger.info("Features are compatible across masters; building later")