diff --git a/pytorch_lightning/trainer/trainer.py b/pytorch_lightning/trainer/trainer.py
index a791d307cf..891ae9764f 100644
--- a/pytorch_lightning/trainer/trainer.py
+++ b/pytorch_lightning/trainer/trainer.py
@@ -803,7 +803,10 @@ class Trainer(TrainerIOMixin,
         # 16 bit mixed precision training using apex
         self.amp_level = amp_level
         self.precision = precision
-        if self.precision == 16:
+
+        assert self.precision == 32 or self.precision == 16, 'only 32 or 16 bit precision supported'
+
+        if self.precision == 16 and num_tpu_cores is None:
             use_amp = True
         self.init_amp(use_amp)