diff --git a/src/emucore/Thumbulator.cxx b/src/emucore/Thumbulator.cxx
index a9d5f3435..bfb835fd5 100644
--- a/src/emucore/Thumbulator.cxx
+++ b/src/emucore/Thumbulator.cxx
@@ -53,7 +53,7 @@ using Common::Base;
 #endif
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-Thumbulator::Thumbulator(const uInt16* rom_ptr, uInt16* ram_ptr, uInt16 rom_size,
+Thumbulator::Thumbulator(const uInt16* rom_ptr, uInt16* ram_ptr, uInt32 rom_size,
                          const uInt32 c_base, const uInt32 c_start, const uInt32 c_stack,
                          bool traponfatal, Thumbulator::ConfigureFor configurefor,
                          Cartridge* cartridge)
@@ -67,7 +67,7 @@ Thumbulator::Thumbulator(const uInt16* rom_ptr, uInt16* ram_ptr, uInt16 rom_size
     configuration(configurefor),
     myCartridge(cartridge)
 {
-  for(uInt16 i = 0; i < romSize / 2; ++i)
+  for(uInt32 i = 0; i < romSize / 2; ++i)
     decodedRom[i] = decodeInstructionWord(CONV_RAMROM(rom[i]));
 
   setConsoleTiming(ConsoleTiming::ntsc);
diff --git a/src/emucore/Thumbulator.hxx b/src/emucore/Thumbulator.hxx
index 964728337..6713ee5c6 100644
--- a/src/emucore/Thumbulator.hxx
+++ b/src/emucore/Thumbulator.hxx
@@ -60,7 +60,7 @@ class Thumbulator
       DPCplus   // cartridges of type DPC+
     };
 
-    Thumbulator(const uInt16* rom_ptr, uInt16* ram_ptr, uInt16 rom_size,
+    Thumbulator(const uInt16* rom_ptr, uInt16* ram_ptr, uInt32 rom_size,
                 const uInt32 c_base, const uInt32 c_start, const uInt32 c_stack,
                 bool traponfatal, Thumbulator::ConfigureFor configurefor,
                 Cartridge* cartridge);
@@ -188,7 +188,7 @@ class Thumbulator
 
   private:
     const uInt16* rom{nullptr};
-    uInt16 romSize{0};
+    uInt32 romSize{0};
     uInt32 cBase{0};
     uInt32 cStart{0};
     uInt32 cStack{0};