diff --git a/u02/include/util.h b/u02/include/util.h
index 383034e..113520d 100644
--- a/u02/include/util.h
+++ b/u02/include/util.h
@@ -33,6 +33,7 @@ Transformation transformation_to_standard_case(int x0, int y0, int x1, int y1);
 
 // Returns the barycentric coordinates of the point given by (xp, yp)
 // in the triangle given by the three points (x0, y0), (x1, y1), (x2, y2).
+// May return non-real values when the points are on a straight line.
 std::tuple<float, float, float> barycentric_coordinates(int x0, int y0, int x1,
                                                         int y1, int x2, int y2,
                                                         int xp, int yp);
diff --git a/u02/src/tests.cpp b/u02/src/tests.cpp
index 87e541e..22c028b 100644
--- a/u02/src/tests.cpp
+++ b/u02/src/tests.cpp
@@ -409,6 +409,17 @@ TEST_CASE("Barycentric coordinates: Edge cases") {
   REQUIRE(b1 == 0);
   REQUIRE(b2 == 0.5);
   REQUIRE(b3 == 0.5);
+
+  // All points on straight line
+  std::tie(b1, b2, b3) = barycentric_coordinates(0, y0, 0, y1, 0, y2, 0, 0);
+  REQUIRE(std::isnan(b1));
+  REQUIRE(std::isnan(b2));
+  REQUIRE(std::isnan(b3));
+
+  std::tie(b1, b2, b3) = barycentric_coordinates(x0, 0, x1, 0, x2, 0, 0, 0);
+  REQUIRE(std::isnan(b1));
+  REQUIRE(std::isnan(b2));
+  REQUIRE(std::isnan(b3));
 }
 
 TEST_CASE("Barycentric coordinates (prop: Σ = 1)") {
@@ -425,7 +436,10 @@ TEST_CASE("Barycentric coordinates (prop: Σ = 1)") {
 
   std::tie(b1, b2, b3) = barycentric_coordinates(x0, y0, x1, y1, x2, y2, x, y);
 
-  REQUIRE_THAT(b1 + b2 + b3, WithinRel(1.0, 1e-4));
+  // If all points are on a straight line, the property does not hold
+  if (!(x0 == x1 && x1 == x2) && !(y0 == y1 && y1 == y2)) {
+    REQUIRE_THAT(b1 + b2 + b3, WithinRel(1.0, 1e-4));
+  }
 }
 
 TEST_CASE("Sort triangle points") {