diff --git a/Machines/Apple/AppleIIgs/MemoryMap.hpp b/Machines/Apple/AppleIIgs/MemoryMap.hpp
index 866952d7c..92033f339 100644
--- a/Machines/Apple/AppleIIgs/MemoryMap.hpp
+++ b/Machines/Apple/AppleIIgs/MemoryMap.hpp
@@ -71,6 +71,7 @@ class MemoryMap {
 
 		bool is_shadowed(const Region &region, uint32_t address) const {
 			const auto physical = physical_address(region, address);
+			assert(physical >= 0 && physical <= 0xff'ffff);
 			return shadow_pages_[(physical >> 10) & 127] & shadow_banks_[physical >> 17];
 		}
 		void write(const Region &region, uint32_t address, uint8_t value) {