diff --git a/CMakeLists.txt b/CMakeLists.txt
index 975f545..1487094 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,7 +3,7 @@ cmake_minimum_required(VERSION 3.1)
 set(CMAKE_CXX_STANDARD 14)
 set(CMAKE_CXX_STANDARD_REQUIRED on)
 
-include_directories(SYSTEM submodules/gsl)
+include_directories(SYSTEM submodules/gsl/include)
 
 add_executable(test_basic test.cpp)
 add_executable(test_graph test_graph.cpp)
@@ -12,5 +12,5 @@ enable_testing()
 
 add_test(test_basic ${CMAKE_BINARY_DIR}/test_basic -s)
 
-# FIXME: Fix graph tests which fail with an assertion error on macOS. 
+# FIXME: Fix graph tests which fail with an assertion error on macOS.
 # add_test(test_graph ${CMAKE_BINARY_DIR}/test_graph -s)
diff --git a/bitflags.h b/bitflags.h
index f2a0c31..817cc40 100644
--- a/bitflags.h
+++ b/bitflags.h
@@ -36,14 +36,16 @@ namespace gcpp {
 	//----------------------------------------------------------------------------
 
 	class bitflags {
+	public:
 		using unit = unsigned int;
 		static_assert(std::is_unsigned<unit>::value, "unit must be an unsigned integral type.");
 
+		static constexpr auto bits_per_unit = static_cast<int>(sizeof(unit) * CHAR_BIT);
+
+	private:
 		std::unique_ptr<unit[]> bits;
 		const int size;
 
-		static constexpr auto bits_per_unit = static_cast<int>(sizeof(unit) * CHAR_BIT);
-
 		//  Return a unit with all bits set if "set" is true, or all bits cleared otherwise.
 		//
 		static constexpr unit all_bits(bool set) noexcept {
@@ -96,9 +98,12 @@ namespace gcpp {
 		//	Test whether all bits are false
 		//
 		bool all_false() const noexcept {
-			auto all_false = [](unit u) { return u == unit(0); };
-			return std::all_of(bits.get(), bits.get() + unit_count(size) - 1, all_false)
-				&& all_false(*(bits.get() + unit_count(size) - 1) & (bit_mask(size) - 1));
+			auto const all_false = [](unit const u) { return u == unit(0); };
+			if (!std::all_of(bits.get(), bits.get() + unit_count(size) - 1, all_false)) {
+				return false;
+			}
+			auto const mask = size % bits_per_unit == 0 ? unit(0) : bit_mask(size);
+			return all_false(*(bits.get() + unit_count(size) - 1) & (mask - 1));
 		}
 
 		//	Set flag value at position
diff --git a/submodules/gsl b/submodules/gsl
index 348a859..8b320e3 160000
--- a/submodules/gsl
+++ b/submodules/gsl
@@ -1 +1 @@
-Subproject commit 348a8595470b2ae7716a99ccacb8189821efb566
+Subproject commit 8b320e3f5d016f953e55dfc7ec8694c1349d3fe4
diff --git a/test.cpp b/test.cpp
index 1d8e85f..ae071ae 100644
--- a/test.cpp
+++ b/test.cpp
@@ -396,7 +396,6 @@ void test_deferred_array() {
 
 }
 
-
 void test_bitflags() {
 	const int N = 100;	// picked so that we have 3 x 32-bit units + 1 partial unit,
 						// so we can exercise the boundary and internal unit cases
@@ -404,7 +403,7 @@ void test_bitflags() {
 	//	Test that we can correctly set any bit range [i,j)
 	for (auto i = 0; i < N; ++i) {
 		for (auto j = i; j < N; ++j) {
-			bitflags flags(100, false);
+			bitflags flags(N, false);
 			flags.set(i, j, true);
 			for (auto test = 0; test < N; ++test) {
 				assert(flags.get(test) == (i <= test && test < j));
@@ -414,7 +413,7 @@ void test_bitflags() {
 
 	//	Test that we can find a true bit set anywhere with any range
 	for (auto set = 0; set < N; ++set) {
-		bitflags flags(100, false);
+		bitflags flags(N, false);
 		flags.set(set, true);
 		for (auto i = 0; i <= set; ++i) {
 			for (auto j = i; j < N; ++j) {
@@ -425,7 +424,7 @@ void test_bitflags() {
 
 	//	Test that we can find a false bit set anywhere with any range
 	for (auto set = 0; set < N; ++set) {
-		bitflags flags(100, true);
+		bitflags flags(N, true);
 		flags.set(set, false);
 		for (auto i = 0; i <= set; ++i) {
 			for (auto j = i; j < N; ++j) {
@@ -434,13 +433,22 @@ void test_bitflags() {
 		}
 	}
 
+	{
+		// Regression test for #23:
+		// Test that all_false observes the bits in the last unit
+		bitflags flags(bitflags::bits_per_unit, false);
+		assert(flags.all_false());
+		flags.set(bitflags::bits_per_unit - 1, true);
+		assert(!flags.all_false());
+	}
+
 	//flags.debug_print();
 }
 
 
 int main() {
 	//test_page();
-	//test_bitflags();
+	test_bitflags();
 
 	//test_deferred_heap();
 	//time_deferred_heap();
@@ -457,7 +465,5 @@ int main() {
 
 	//heap.collect();
 	//heap.debug_print();
-
-	return 0;
 }