Index: src/of_asprintf.m
==================================================================
--- src/of_asprintf.m
+++ src/of_asprintf.m
@@ -304,12 +304,13 @@
 		ctx->lengthModifier = LENGTH_MODIFIER_J;
 
 		break;
 	case 'z':
 #if defined(OF_WINDOWS)
-		if (!appendSubformat(ctx, "I", 1))
-			return false;
+		if (sizeof(size_t) == 8)
+			if (!appendSubformat(ctx, "I64", 3))
+				return false;
 #elif defined(_NEWLIB_VERSION)
 		if (!appendSubformat(ctx, "l", 1))
 			return false;
 #else
 		if (!appendSubformat(ctx, ctx->format + ctx->i, 1))
@@ -319,12 +320,13 @@
 		ctx->lengthModifier = LENGTH_MODIFIER_Z;
 
 		break;
 	case 't':
 #if defined(OF_WINDOWS)
-		if (!appendSubformat(ctx, "I", 1))
-			return false;
+		if (sizeof(ptrdiff_t) == 8)
+			if (!appendSubformat(ctx, "I64", 3))
+				return false;
 #elif defined(_NEWLIB_VERSION)
 		if (!appendSubformat(ctx, "l", 1))
 			return false;
 #else
 		if (!appendSubformat(ctx, ctx->format + ctx->i, 1))