diff --git a/TTS/api.py b/TTS/api.py
index 12e82af5..ed828250 100644
--- a/TTS/api.py
+++ b/TTS/api.py
@@ -357,15 +357,17 @@ class TTS(nn.Module):
             target_wav (str):`
                 Path to the target wav file.
         """
-        wav = self.voice_converter.voice_conversion(source_wav=source_wav, target_wav=target_wav)
-        return wav
+        if self.voice_converter is None:
+            msg = "The selected model does not support voice conversion."
+            raise RuntimeError(msg)
+        return self.voice_converter.voice_conversion(source_wav=source_wav, target_wav=target_wav)
 
     def voice_conversion_to_file(
         self,
         source_wav: str,
         target_wav: str,
         file_path: str = "output.wav",
-    ):
+    ) -> str:
         """Voice conversion with FreeVC. Convert source wav to target speaker.
 
         Args: