diff --git a/node_helpers.py b/node_helpers.py
index cac4e88dd..7f431f91b 100644
--- a/node_helpers.py
+++ b/node_helpers.py
@@ -86,6 +86,7 @@ def image_alpha_fix(destination, source):
     if destination.shape[-1] < source.shape[-1]:
         source = source[...,:destination.shape[-1]]
     elif destination.shape[-1] > source.shape[-1]:
-        source = torch.nn.functional.pad(source, (0, 1))
-        source[..., -1] = 1.0
+        pad = destination.shape[-1] - source.shape[-1]
+        source = torch.nn.functional.pad(source, (0, pad))
+        source[..., -pad:] = 1.0
     return destination, source
diff --git a/tests-unit/node_helpers_test.py b/tests-unit/node_helpers_test.py
new file mode 100644
index 000000000..edd12e88b
--- /dev/null
+++ b/tests-unit/node_helpers_test.py
@@ -0,0 +1,27 @@
+import torch
+
+import node_helpers
+
+
+class TestImageAlphaFix:
+    def test_pads_rgb_to_rgba(self):
+        destination = torch.zeros(1, 2, 2, 4)
+        source = torch.full((1, 2, 2, 3), 0.5)
+        _, source = node_helpers.image_alpha_fix(destination, source)
+        assert source.shape[-1] == 4
+        assert torch.all(source[..., :3] == 0.5)  # original channels preserved
+        assert torch.all(source[..., 3] == 1.0)   # added alpha
+
+    def test_pads_more_than_one_channel(self):
+        # channel gap > 1 (grayscale source, RGBA destination) must still match up.
+        destination = torch.zeros(1, 2, 2, 4)
+        source = torch.zeros(1, 2, 2, 1)
+        _, source = node_helpers.image_alpha_fix(destination, source)
+        assert source.shape[-1] == 4
+        assert torch.all(source[..., 1:] == 1.0)
+
+    def test_truncates_when_source_has_more_channels(self):
+        destination = torch.zeros(1, 2, 2, 3)
+        source = torch.ones(1, 2, 2, 4)
+        _, source = node_helpers.image_alpha_fix(destination, source)
+        assert source.shape[-1] == 3