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/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 +50,54 @@
 		}
 	}
 
+	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;
+
+
+		for (int j = 0; j < 16; ++j)
+			TS_ASSERT_DELTA(a._data[j], n._data[j], 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;
+
+		bool different = false;
+		for (int j = 0; j < 16; ++j)
+		{
+			if (!feq(b._data[j], n._data[j], EPS))
+			{
+				different = true;
+				break;
+			}
+		}
+		TS_ASSERT(different);
+	}
+
 	void test_quats()
 	{
 		srand(0);