diff --git a/pxtcompiler/emitter/hexfile.ts b/pxtcompiler/emitter/hexfile.ts
index ec2571b98b36..3d9c7fde8627 100644
--- a/pxtcompiler/emitter/hexfile.ts
+++ b/pxtcompiler/emitter/hexfile.ts
@@ -1019,9 +1019,11 @@ ${hexfile.hexPrelude()}
         let b = mkProcessorFile(target)
         b.emit(src);
 
+        let flashUsableEnd = target.flashUsableEnd ? target.flashUsableEnd : target.flashEnd
+
         src = `; Interface tables: ${bin.itFullEntries}/${bin.itEntries} (${Math.round(100 * bin.itFullEntries / bin.itEntries)}%)\n` +
             `; Virtual methods: ${bin.numVirtMethods} / ${bin.numMethods}\n` +
-            b.getSource(!peepDbg, bin.numStmts, target.flashUsableEnd);
+            b.getSource(!peepDbg, bin.numStmts, flashUsableEnd);
 
         throwAssemblerErrors(b)
 
diff --git a/pxtlib/emitter/assembler.ts b/pxtlib/emitter/assembler.ts
index 2815c25f4a98..0b0ad3f4e694 100644
--- a/pxtlib/emitter/assembler.ts
+++ b/pxtlib/emitter/assembler.ts
@@ -1084,7 +1084,7 @@ namespace ts.pxtc.assembler {
             let totalEnd = (lenTotal + this.baseOffset - 1) & 0xffffff
 
             if (flashUsableEnd && totalEnd >= flashUsableEnd) {
-                const e = new Error(lf("program too big by {0} bytes!", totalEnd - flashUsableEnd));
+                const e = new Error(lf("program too big by {0} bytes!", totalEnd - flashUsableEnd + 1));
                 (e as any).ksErrorCode = 9283;
                 throw e;
             }