Index: source/gui/ObjectTypes/CMiniMap.cpp
===================================================================
--- source/gui/ObjectTypes/CMiniMap.cpp
+++ source/gui/ObjectTypes/CMiniMap.cpp
@@ -608,7 +608,7 @@
 	// Move the minimap to it's final location.
 	unitMatrix.Translate(x, y, 0.0f);
 	// Apply the gui matrix.
-	unitMatrix *= GetDefaultGuiMatrix();
+	unitMatrix.Concatenate(GetDefaultGuiMatrix());
 	// Load the transform into the shader.
 	shader->Uniform(str_transform, unitMatrix);
 
Index: source/lib/self_test.h
===================================================================
--- source/lib/self_test.h
+++ source/lib/self_test.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2019 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  *
  * Permission is hereby granted, free of charge, to any person obtaining
  * a copy of this software and associated documentation files (the
@@ -24,8 +24,9 @@
 #define INCLUDED_SELF_TEST
 
 // for convenience, to avoid having to include all of these manually
-#include "lib/status.h"
+#include "lib/lib.h"
 #include "lib/os_path.h"
+#include "lib/status.h"
 #include "lib/posix/posix.h"
 
 #define CXXTEST_HAVE_EH
@@ -147,6 +148,14 @@
 #define TS_ASSERT_VECTOR_EQUALS_ARRAY(vec1, array) TS_ASSERT_EQUALS(vec1, ts_make_vector((array), sizeof(array)))
 #define TS_ASSERT_VECTOR_CONTAINS(vec1, element) TS_ASSERT(std::find((vec1).begin(), (vec1).end(), element) != (vec1).end());
 
+#define TS_ASSERT_MATRIX_EQUALS_DELTA(m1, m2, size, epsilon)  \
+		for (int j = 0; j < size; ++j)						  \
+			TS_ASSERT_DELTA(m1._data[j], m2._data[j], epsilon);
+
+#define TS_ASSERT_MATRIX_DIFFERS_DELTA(m1, m2, size, epsilon)  \
+		for (int j = 0; j < size; ++j)						  \
+			TS_ASSERT(!feq(m1._data[j], m2._data[j], epsilon));
+
 class ScriptInterface;
 // Script-based testing setup (defined in test_setup.cpp). Defines TS_* functions.
 void ScriptTestSetup(const ScriptInterface&);
Index: source/maths/Matrix3D.h
===================================================================
--- source/maths/Matrix3D.h
+++ source/maths/Matrix3D.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2019 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -126,8 +126,7 @@
 	// matrix multiplication/assignment
 	CMatrix3D& operator*=(const CMatrix3D &matrix)
 	{
-		Concatenate(matrix);
-		return *this;
+		return *this = *this * matrix;
 	}
 
 	// matrix scaling
Index: source/maths/tests/test_Matrix3d.h
===================================================================
--- source/maths/tests/test_Matrix3d.h
+++ source/maths/tests/test_Matrix3d.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2011 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -50,6 +50,44 @@
 		}
 	}
 
+	void test_compoundMultiplication()
+	{
+		const float EPS = 2e-4f;
+		float invertibleData[16] = {
+			2.f, -3.f, 0.f, 1.f,
+			-2.f, 3.f, 0.f, 0.f,
+			1.f, -2.f, 1.f, 0.f,
+			1.f, -1.f, 0.f, 0.f
+		};
+
+		// Invertible matrix.
+		CMatrix3D a(invertibleData);
+		CMatrix3D n;
+		a.GetInverse(n);
+		a *= n;
+		CMatrix3D a2(invertibleData);
+		n *= a2;
+
+
+		TS_ASSERT_MATRIX_EQUALS_DELTA(a, n, 16, EPS);
+
+		// Non invertible matrix.
+		float nonInvertibleData[16] = {
+			2.f, -3.f, 0.f, 1.f,
+			-2.f, 3.f, 0.f, 0.f,
+			1.f, -2.f, 1.f, 0.f,
+			0.f, 0.f, 0.f, 0.f
+		};
+
+		CMatrix3D b(nonInvertibleData);
+		b.GetInverse(n);
+		b *= n;
+		CMatrix3D b2(nonInvertibleData);
+		n *= b2;
+
+		TS_ASSERT_MATRIX_DIFFERS_DELTA(b, n, 16, EPS);
+	}
+
 	void test_quats()
 	{
 		srand(0);