diff --git a/src/testrand_impl.h b/src/testrand_impl.h
index e643778f36b1e..c8d30ef6a814b 100644
--- a/src/testrand_impl.h
+++ b/src/testrand_impl.h
@@ -127,7 +127,7 @@ static void secp256k1_testrand_init(const char* hexseed) {
             pos++;
         }
     } else {
-        FILE *frand = fopen("/dev/urandom", "r");
+        FILE *frand = fopen("/dev/urandom", "rb");
         if ((frand == NULL) || fread(&seed16, 1, sizeof(seed16), frand) != sizeof(seed16)) {
             uint64_t t = time(NULL) * (uint64_t)1337;
             fprintf(stderr, "WARNING: could not read 16 bytes from /dev/urandom; falling back to insecure PRNG\n");