diff --git a/src/diffusers/models/controlnets/controlnet_flux.py b/src/diffusers/models/controlnets/controlnet_flux.py
index b619bfa652b6..9041b09649cc 100644
--- a/src/diffusers/models/controlnets/controlnet_flux.py
+++ b/src/diffusers/models/controlnets/controlnet_flux.py
@@ -430,7 +430,7 @@ def forward(
     ) -> Union[FluxControlNetOutput, Tuple]:
         # ControlNet-Union with multiple conditions
         # only load one ControlNet for saving memories
-        if len(self.nets) == 1 and self.nets[0].union:
+        if len(self.nets) == 1:
             controlnet = self.nets[0]
 
             for i, (image, mode, scale) in enumerate(zip(controlnet_cond, controlnet_mode, conditioning_scale)):
@@ -454,17 +454,18 @@ def forward(
                     control_block_samples = block_samples
                     control_single_block_samples = single_block_samples
                 else:
-                    control_block_samples = [
-                        control_block_sample + block_sample
-                        for control_block_sample, block_sample in zip(control_block_samples, block_samples)
-                    ]
-
-                    control_single_block_samples = [
-                        control_single_block_sample + block_sample
-                        for control_single_block_sample, block_sample in zip(
-                            control_single_block_samples, single_block_samples
-                        )
-                    ]
+                    if block_samples is not None and control_block_samples is not None:
+                        control_block_samples = [
+                            control_block_sample + block_sample
+                            for control_block_sample, block_sample in zip(control_block_samples, block_samples)
+                        ]
+                    if single_block_samples is not None and control_single_block_samples is not None:
+                        control_single_block_samples = [
+                            control_single_block_sample + block_sample
+                            for control_single_block_sample, block_sample in zip(
+                                control_single_block_samples, single_block_samples
+                            )
+                        ]
 
         # Regular Multi-ControlNets
         # load all ControlNets into memories