--- tools/tiff2pdf.c
+++ tools/tiff2pdf.c
@@ -3668,7 +3668,7 @@
 	written += TIFFWriteFile(output, (tdata_t) "(", 1);
 	for (i=0;i<len;i++){
 		if((pdfstr[i]&0x80) || (pdfstr[i]==127) || (pdfstr[i]<32)){
-			sprintf(buffer, "\\%.3o", pdfstr[i]);
+			sprintf(buffer, "\\%.3o", (unsigned char)(pdfstr[i]));
 			written += TIFFWriteFile(output, (tdata_t) buffer, 4);
 		} else {
 			switch (pdfstr[i]){