diff --git a/src/fart/Cli.cpp b/src/fart/Cli.cpp
index d4fd149..ca931fc 100644
--- a/src/fart/Cli.cpp
+++ b/src/fart/Cli.cpp
@@ -2,6 +2,8 @@
 #include <string.h>
 #include <stdlib.h>
 #include <stdio.h>
+#include <errno.h>
+#include <stdint.h>
 
 #define FLAGS \
     FLAG(height, "h", true) \
@@ -137,13 +139,41 @@ int Cli::run(int argc, char * argv[])
     return 0;
 }
 
+static bool parse_size(const char * flag, const char * value, size_t * out)
+{
+    bool valid = value[0] != '\0';
+    for (const char * c = value; *c != '\0'; c++)
+    {
+        if (*c < '0' || *c > '9')
+        {
+            valid = false;
+            break;
+        }
+    }
+    if (valid)
+    {
+        errno = 0;
+        unsigned long long v = strtoull(value, nullptr, 10);
+        if (errno == 0 && v <= SIZE_MAX)
+        {
+            *out = (size_t)v;
+            return true;
+        }
+    }
+    fprintf(stderr, "Error: invalid value for option %s: '%s'\n", flag, value);
+    return false;
+}
+
 int Cli::handle_option(size_t flag_idx, const char * arg_value)
 {
     switch (flag_idx)
     {
     case flag_height:
     case flag_height_l:
-        this->options.height = atoi(arg_value);
+        if (!parse_size(flag_defs[flag_idx].flag, arg_value, &this->options.height))
+        {
+            return 2;
+        }
         return 0;
 
     case flag_help:
@@ -165,7 +195,10 @@ int Cli::handle_option(size_t flag_idx, const char * arg_value)
 
     case flag_width:
     case flag_width_l:
-        this->options.width = atoi(arg_value);
+        if (!parse_size(flag_defs[flag_idx].flag, arg_value, &this->options.width))
+        {
+            return 2;
+        }
         return 0;
 
     case flag_version: