diff --git a/client/fpga_compress.c b/client/fpga_compress.c
index 9bafead9a..6af1846ef 100644
--- a/client/fpga_compress.c
+++ b/client/fpga_compress.c
@@ -77,7 +77,7 @@ int zlib_compress(FILE *infile[], uint8_t num_infiles, FILE *outfile)
 	uint8_t *fpga_config;
 	uint32_t i;
 	int ret;
-	uint8_t c;		
+	int c;		
 	z_stream compressed_fpga_stream;
 
 	fpga_config = malloc(num_infiles * FPGA_CONFIG_SIZE);
@@ -99,7 +99,7 @@ int zlib_compress(FILE *infile[], uint8_t num_infiles, FILE *outfile)
 			for(uint16_t k = 0; k < FPGA_INTERLEAVE_SIZE; k++) {
 				c = fgetc(infile[j]);
 				if (!feof(infile[j])) {
-					fpga_config[i++] = c;
+					fpga_config[i++] = c &0xFF;
 				} else if (num_infiles > 1) {
 					fpga_config[i++] = '\0';
 				}