diff --git a/tdd_for_embedded_c/chapter04/Makefile b/tdd_for_embedded_c/chapter04/Makefile
index 62e823d..e9c6c6b 100644
--- a/tdd_for_embedded_c/chapter04/Makefile
+++ b/tdd_for_embedded_c/chapter04/Makefile
@@ -48,7 +48,8 @@ SRC_FILES=\
 	../test/LedDriverTest.c \
 	../test/LedDriverTestRunner.c \
 	../test/AllTests.c \
-	../src/LedDriver.c
+	../src/LedDriver.c \
+	../mocks/RuntimeErrorStub.c
 
 INC_DIRS=-I$(UNITY_ROOT)/src -I$(UNITY_ROOT)/extras/fixture/src
 SYMBOLS=
diff --git a/tdd_for_embedded_c/chapter04/mocks/RuntimeErrorStub.c b/tdd_for_embedded_c/chapter04/mocks/RuntimeErrorStub.c
new file mode 100644
index 0000000..9562a4f
--- /dev/null
+++ b/tdd_for_embedded_c/chapter04/mocks/RuntimeErrorStub.c
@@ -0,0 +1,33 @@
+#include "RuntimeErrorStub.h"
+#include "../util/RuntimeError.h"
+
+
+static const char *message = "No Error";
+static int parameter = -1;
+static const char *file = 0;
+static int line = -1;
+
+
+void RuntimeErrorStub_Reset(void) {
+  message = "No Error";
+  parameter = -1;
+}
+
+
+const char *RuntimeErrorStub_GetLastError(void) {
+  return message;
+}
+
+
+void RuntimeError(const char *m, int p,
+                  const char *f, int l) {
+  message = m;
+  parameter = p;
+  file = f;
+  line = l;
+}
+
+
+int RuntimeErrorStub_GetLastParameter(void) {
+  return parameter;
+}
diff --git a/tdd_for_embedded_c/chapter04/mocks/RuntimeErrorStub.h b/tdd_for_embedded_c/chapter04/mocks/RuntimeErrorStub.h
new file mode 100644
index 0000000..d2756ea
--- /dev/null
+++ b/tdd_for_embedded_c/chapter04/mocks/RuntimeErrorStub.h
@@ -0,0 +1,3 @@
+void RuntimeErrorStub_Reset(void);
+const char *RuntimeErrorStub_GetLastError(void);
+int RuntimeErrorStub_GetLastParameter(void);
diff --git a/tdd_for_embedded_c/chapter04/util/RuntimeError.h b/tdd_for_embedded_c/chapter04/util/RuntimeError.h
new file mode 100644
index 0000000..8970f37
--- /dev/null
+++ b/tdd_for_embedded_c/chapter04/util/RuntimeError.h
@@ -0,0 +1,6 @@
+void RuntimeError(const char *m, int p,
+                  const char *f, int l);
+
+
+#define RUNTIME_ERROR(description, parameter) \
+  RuntimeError(description, parameter, __FILE__, __LINE__)
\ No newline at end of file