diff --git a/src/common.cu b/src/common.cu
index ff4e1fd..25fc7da 100644
--- a/src/common.cu
+++ b/src/common.cu
@@ -40,22 +40,40 @@ static int ncclroot = 0;
 static int parallel_init = 0;
 static int blocking_coll = 0;
 
-double parsesize(char *value) {
+static double parsesize(const char *value) {
     long long int units;
     double size;
+    char size_lit;
 
-    if (strchr(value, 'G') != NULL) {
-        units=1024*1024*1024;
-    } else if (strchr(value, 'M') != NULL) {
-        units=1024*1024;
-    } else if (strchr(value, 'K') != NULL) {
-        units=1024;
-    } else {
-        units=1;
+    int count = sscanf(value, "%lf %1s", &size, &size_lit);
+
+    switch (count) {
+    case 2:
+      switch (size_lit) {
+      case 'G':
+      case 'g':
+        units = 1024*1024*1024;
+        break;
+      case 'M':
+      case 'm':
+        units = 1024*1024;
+        break;
+      case 'K':
+      case 'k':
+        units = 1024;
+        break;
+      default:
+        return -1.0;
+      };
+      break;
+    case 1:
+      units = 1;
+      break;
+    default:
+      return -1.0;
     }
 
-    size = atof(value)*units;
-    return size;
+    return size * units;
 }
 
 double DeltaMaxValue(ncclDataType_t type) {
@@ -570,6 +588,7 @@ int main(int argc, char* argv[]) {
   setlinebuf(stdout);
 
   // Parse args
+  double parsed;
   int longindex;
   static struct option longopts[] = {
     {"nthreads", required_argument, 0, 't'},
@@ -605,10 +624,20 @@ int main(int argc, char* argv[]) {
         nGpus = strtol(optarg, NULL, 0);
         break;
       case 'b':
-        minBytes = (size_t)parsesize(optarg);
+        parsed = parsesize(optarg);
+        if (parsed < 0) {
+          fprintf(stderr, "invalid size specified for 'minbytes'\n");
+          return -1;
+        }
+        minBytes = (size_t)parsed;
         break;
       case 'e':
-        maxBytes = (size_t)parsesize(optarg);
+        parsed = parsesize(optarg);
+        if (parsed < 0) {
+          fprintf(stderr, "invalid size specified for 'maxbytes'\n");
+          return -1;
+        }
+        maxBytes = (size_t)parsed;
         break;
       case 'i':
         stepBytes = strtol(optarg, NULL, 0);
@@ -623,7 +652,7 @@ int main(int argc, char* argv[]) {
 #if NCCL_MAJOR >= 2 && NCCL_MINOR >= 2
         agg_iters = (int)strtol(optarg, NULL, 0);
 #else
-        printf("Option -m not supported before NCCL 2.2. Ignoring\n");
+        fprintf(stderr, "Option -m not supported before NCCL 2.2. Ignoring\n");
 #endif
         break;
       case 'w':
@@ -648,7 +677,7 @@ int main(int argc, char* argv[]) {
         blocking_coll = strtol(optarg, NULL, 0);
         break;
       case 'h':
-	printf("USAGE: %s \n\t"
+        fprintf(stderr, "USAGE: %s \n\t"
             "[-t,--nthreads <num threads>] \n\t"
             "[-g,--ngpus <gpus per thread>] \n\t"
             "[-b,--minbytes <min size in bytes>] \n\t"
@@ -668,8 +697,8 @@ int main(int argc, char* argv[]) {
 	    basename(argv[0]));
 	return 0;
       default:
-        printf("invalid option \n");
-	printf("USAGE: %s \n\t"
+        fprintf(stderr, "invalid option \n");
+        fprintf(stderr, "USAGE: %s \n\t"
             "[-t,--nthreads <num threads>] \n\t"
             "[-g,--ngpus <gpus per thread>] \n\t"
             "[-b,--minbytes <min size in bytes>] \n\t"
@@ -690,6 +719,12 @@ int main(int argc, char* argv[]) {
 	return 0;
     }
   }
+  if (minBytes > maxBytes) {
+    fprintf(stderr, "invalid sizes for 'minbytes' and 'maxbytes': %llu > %llu\n",
+           (unsigned long long)minBytes,
+           (unsigned long long)maxBytes);
+    return -1;
+  }
 #ifdef MPI_SUPPORT
   MPI_Init(&argc, &argv);
 #endif