diff --git a/src/odl_renderer/colors.py b/src/odl_renderer/colors.py
index d9a200c..4de3dc1 100644
--- a/src/odl_renderer/colors.py
+++ b/src/odl_renderer/colors.py
@@ -1,3 +1,19 @@
+import logging
+
+_LOGGER = logging.getLogger(__name__)
+
+# Tokens we've already warned about, so an unknown color in a dashboard that
+# re-renders on every update logs once rather than on every frame.
+_warned_color_tokens: set[str] = set()
+
+
+def _warn_unresolved_color(token: str, reason: str) -> None:
+    """Log once that *token* could not be resolved and will render as white."""
+    if token not in _warned_color_tokens:
+        _warned_color_tokens.add(token)
+        _LOGGER.warning("Color %r %s; rendering as white", token, reason)
+
+
 # Color constants with alpha channel
 WHITE = (255, 255, 255, 255)
 BLACK = (0, 0, 0, 255)
@@ -115,8 +131,10 @@ def _parse_hex(hex_val: str) -> tuple[int, int, int, int]:
                 b = int(hex_val[4:6], 16)
                 a = int(hex_val[6:8], 16)
             else:
+                _warn_unresolved_color(f"#{hex_val}", "is not a valid hex color (expected 3, 4, 6, or 8 digits)")
                 return WHITE
         except ValueError:
+            _warn_unresolved_color(f"#{hex_val}", "contains non-hex characters")
             return WHITE
         return r, g, b, a
 
@@ -131,4 +149,8 @@ def _resolve_named(self, color_str: str) -> tuple[int, int, int, int]:
             return YELLOW if self.accent_color == "yellow" else RED
         if color_str in ("half_accent", "ha"):
             return HALF_YELLOW if self.accent_color == "yellow" else HALF_RED
-        return NAMED_COLORS.get(color_str, WHITE)
+        resolved = NAMED_COLORS.get(color_str)
+        if resolved is None:
+            _warn_unresolved_color(color_str, "is not a known color name")
+            return WHITE
+        return resolved
diff --git a/src/odl_renderer/elements/shapes.py b/src/odl_renderer/elements/shapes.py
index 77150fd..60fd369 100644
--- a/src/odl_renderer/elements/shapes.py
+++ b/src/odl_renderer/elements/shapes.py
@@ -299,6 +299,14 @@ def draw_dashed_line(
     dy = y2 - y1
     line_length = (dx**2 + dy**2) ** 0.5
 
+    # A zero-length line (start == end, easily produced by templated coordinates)
+    # has no direction to step along and would divide by zero below. Draw the single
+    # point if a dash starts there, then stop.
+    if line_length == 0:
+        if dash_length > 0:
+            draw.line([(x1, y1), (x2, y2)], fill=fill, width=width)
+        return
+
     step_x = dx / line_length
     step_y = dy / line_length
 
diff --git a/tests/integration/test_shapes.py b/tests/integration/test_shapes.py
index 3e2855f..9c3bca2 100644
--- a/tests/integration/test_shapes.py
+++ b/tests/integration/test_shapes.py
@@ -25,6 +25,15 @@ async def test_dashed_line(self):
         )
         assert image.size == (200, 100)
 
+    async def test_zero_length_dashed_line_does_not_crash(self):
+        """A dashed line with start == end must not divide by zero (finding A4)."""
+        image = await generate_image(
+            width=200,
+            height=100,
+            elements=[E.line(x_start=50, y_start=50, x_end=50, y_end=50, dashed=True)],
+        )
+        assert image.size == (200, 100)
+
     async def test_line_colors(self):
         for color in ["black", "red", "#FF0000"]:
             image = await generate_image(
diff --git a/tests/unit/test_colors.py b/tests/unit/test_colors.py
index f4eef91..7ec56a3 100644
--- a/tests/unit/test_colors.py
+++ b/tests/unit/test_colors.py
@@ -143,3 +143,41 @@ def test_accent_color_default_red(self):
         resolver = ColorResolver()
         assert resolver.resolve("accent") == RED
         assert resolver.resolve("half_accent") == HALF_RED
+
+
+class TestUnresolvedColorWarnings:
+    """Unknown names and malformed hex render white but should warn once (finding A11)."""
+
+    def test_unknown_name_warns_once(self, caplog):
+        import logging
+
+        from odl_renderer import colors
+
+        colors._warned_color_tokens.discard("balck")
+        resolver = ColorResolver()
+        with caplog.at_level(logging.WARNING, logger="odl_renderer.colors"):
+            assert resolver.resolve("balck") == WHITE
+            assert resolver.resolve("balck") == WHITE  # second call must not re-warn
+        warnings = [r for r in caplog.records if "balck" in r.getMessage()]
+        assert len(warnings) == 1
+
+    def test_malformed_hex_warns_once(self, caplog):
+        import logging
+
+        from odl_renderer import colors
+
+        colors._warned_color_tokens.discard("#12345")
+        resolver = ColorResolver()
+        with caplog.at_level(logging.WARNING, logger="odl_renderer.colors"):
+            assert resolver.resolve("#12345") == WHITE
+            assert resolver.resolve("#12345") == WHITE
+        warnings = [r for r in caplog.records if "#12345" in r.getMessage()]
+        assert len(warnings) == 1
+
+    def test_known_color_does_not_warn(self, caplog):
+        import logging
+
+        resolver = ColorResolver()
+        with caplog.at_level(logging.WARNING, logger="odl_renderer.colors"):
+            assert resolver.resolve("red") == RED
+        assert not caplog.records