diff --git a/src/glayout/primitives/mimcap.py b/src/glayout/primitives/mimcap.py
index 5d6e24d2..c7c4464a 100644
--- a/src/glayout/primitives/mimcap.py
+++ b/src/glayout/primitives/mimcap.py
@@ -289,7 +289,8 @@ def mimcap(
                         layer=pdk.get_glayer(capmetbottom),
                         centered=True
                         )
-                appendix_pos=target_port.center
+                # Port.center is a tuple; the component below indexes into it.
+                appendix_pos=list(target_port.center)
                 appendix_pos[cord_ref] = appendix_pos[cord_ref] + appendix_size[cord_ref]/2 if direction=="N" or direction=="E" else appendix_pos[cord_ref] - appendix_size[cord_ref]/2
                 appendix.move(appendix_pos)
     
diff --git a/tests/test_mimcap_build.py b/tests/test_mimcap_build.py
new file mode 100644
index 00000000..065774fa
--- /dev/null
+++ b/tests/test_mimcap_build.py
@@ -0,0 +1,84 @@
+"""The mimcap primitive builds on its own, not only through mimcap_array.
+
+`mimcap_array` passes `with_extension=False`, so every cell in the suite
+reaches the primitive through the one path that skips the appendix code. A
+direct `mimcap(pdk, size)` -- the obvious way to use it -- goes down the
+other branch, and that is where it raised `TypeError: 'tuple' object does
+not support item assignment`.
+
+The appendix branch is entered only when the requested size leaves
+`min_via_distance < 0.4`, so the size matters: 5x5 reaches it. Both MIM
+options are covered because they build different stacks (A on
+met2/FuseTop/met3, B on met4/FuseTop/met5) around the same code.
+"""
+import unittest
+
+
+class MimcapBuildTests(unittest.TestCase):
+    SIZES = ((5.0, 5.0), (10.0, 4.0))   # square hits the appendix branch
+
+    def _pdk(self):
+        from glayout.pdk.gf180_mapped.gf180_mapped import gf180_mapped_pdk
+        return gf180_mapped_pdk
+
+    def test_mimcap_builds_with_extensions(self):
+        """The default path. This is the one that regressed."""
+        from glayout.primitives.mimcap import mimcap
+        pdk = self._pdk()
+        for option in ("A", "B"):
+            for size in self.SIZES:
+                with self.subTest(option=option, size=size):
+                    comp = mimcap(pdk, size=size, option=option)
+                    self.assertTrue(comp.get_ports_list())
+
+    def test_mimcap_builds_without_extensions(self):
+        """The path mimcap_array takes, which never regressed.
+
+        Kept so a future change that fixes one branch by breaking the other
+        does not look like a pass.
+        """
+        from glayout.primitives.mimcap import mimcap
+        pdk = self._pdk()
+        for option in ("A", "B"):
+            with self.subTest(option=option):
+                comp = mimcap(pdk, size=(5.0, 5.0), option=option,
+                              with_extension=False)
+                self.assertTrue(comp.get_ports_list())
+
+    def test_mimcap_array_builds(self):
+        from glayout.primitives.mimcap import mimcap_array
+        comp = mimcap_array(self._pdk(), rows=2, columns=2, size=(5.0, 5.0))
+        self.assertTrue(comp.get_ports_list())
+
+    def test_plates_land_on_the_layers_the_option_names(self):
+        """Option A is a met2/met3 stack, option B a met4/met5 one.
+
+        Building is not enough: the two options differ only in which metals
+        they draw, so a fix that builds both while drawing the same stack
+        would pass every check above.
+
+        Read back from the written GDS rather than from the Component, so
+        this does not depend on which backend is active.
+        """
+        import gdstk
+        import tempfile
+        from glayout.primitives.mimcap import mimcap
+        pdk = self._pdk()
+        expected = {"A": ("met2", "met3"), "B": ("met4", "met5")}
+        for option, glayers in expected.items():
+            with self.subTest(option=option):
+                comp = mimcap(pdk, size=(5.0, 5.0), option=option)
+                with tempfile.TemporaryDirectory() as tmp:
+                    path = tmp + "/mimcap.gds"
+                    comp.write_gds(path)
+                    top = gdstk.read_gds(path).top_level()[0]
+                    top.flatten()
+                    drawn = {(p.layer, p.datatype) for p in top.polygons}
+                for glayer in glayers:
+                    layer, datatype = pdk.get_glayer(glayer)
+                    self.assertIn((int(layer), int(datatype)), drawn,
+                                  "option %s should draw %s" % (option, glayer))
+
+
+if __name__ == "__main__":
+    unittest.main()