diff --git a/Machines/Apple/AppleIIgs/MemoryMap.hpp b/Machines/Apple/AppleIIgs/MemoryMap.hpp
index 724f7c908..cf3f8908f 100644
--- a/Machines/Apple/AppleIIgs/MemoryMap.hpp
+++ b/Machines/Apple/AppleIIgs/MemoryMap.hpp
@@ -379,6 +379,7 @@ class MemoryMap {
 					c0_region.flags |= Region::IsIO;
 
 #define apply_region(flag, region)	\
+					region.write = nullptr;	\
 					if(flag) {	\
 						region.read = rom;	\
 						region.flags &= ~Region::IsIO;	\
diff --git a/OSBindings/Mac/Clock SignalTests/IIgsMemoryMapTests.mm b/OSBindings/Mac/Clock SignalTests/IIgsMemoryMapTests.mm
index ab9a1f831..06e17c41c 100644
--- a/OSBindings/Mac/Clock SignalTests/IIgsMemoryMapTests.mm	
+++ b/OSBindings/Mac/Clock SignalTests/IIgsMemoryMapTests.mm	
@@ -318,9 +318,13 @@ namespace {
 		// Test read pointers.
 		testMemory(@"read", ^(int logical, int physical, const MemoryMap::Region &region) {
 			XCTAssert(region.read != nullptr);
-			const int foundPhysical = physicalOffset(&region.read[logical << 8]);
+			if(region.read == nullptr) {
+				*stop = YES;
+				return;
+			}
 
 			// Compare to correct value.
+			const int foundPhysical = physicalOffset(&region.read[logical << 8]);
 			XCTAssert(physical == foundPhysical,
 				@"Logical page %04x should be mapped to read physical %04x; is instead %04x",
 					logical,
@@ -341,9 +345,13 @@ namespace {
 			}
 
 			XCTAssert(region.write != nullptr);
-			const int foundPhysical = physicalOffset(&region.write[logical << 8]);
+			if(region.write == nullptr) {
+				*stop = YES;
+				return;
+			}
 
 			// Compare to correct value.
+			const int foundPhysical = physicalOffset(&region.write[logical << 8]);
 			XCTAssert(physical == foundPhysical,
 				@"Logical page %04x should be mapped to write physical %04x; is instead %04x",
 					logical,