diff --git a/Makefile b/Makefile
index edca96b..30b6a31 100644
--- a/Makefile
+++ b/Makefile
@@ -56,11 +56,15 @@ build-test: $(BINS_TEST)
 $(BINS_TEST): %: %.c.o $(OBJS)
 	$(CC) $^ -o $@
 
-# Each test includes the test directory, which contains the acutest header file
+# Allow with the include directory, each test includes $(TEST_DIR) (which
+# contains the acutest.h header file), and the src directory of the module it's
+# testing. This allows tests to access internal methods, which aren't publicly
+# exposed.
 $(BUILD_DIR)/$(TEST_DIR)/%.c.o: $(TEST_DIR)/%.c
 	mkdir -p $(dir $@)
-	$(CC) $(CFLAGS) -I$(TEST_DIR) -c $< -o $@
-
+	$(CC) $(CFLAGS) -I$(TEST_DIR) \
+		-I$(dir $(@:$(BUILD_DIR)/$(TEST_DIR)/%=$(SRC_DIR)/%)) \
+		-c $< -o $@
 
 # =====MAINTENANCE=====
 .PHONY: lint
diff --git a/test/heap/test_heap.c b/test/heap/test_heap.c
index 4079569..832404d 100644
--- a/test/heap/test_heap.c
+++ b/test/heap/test_heap.c
@@ -1,5 +1,6 @@
 #include "acutest.h"
 #include "vieter_heap.h"
+#include "vieter_heap_tree.h"
 
 #define TEST_SIZE(heap, size) \
     TEST_CHECK(vieter_heap_size(heap) == size); \