diff --git a/07/solution.c b/07/solution.c
index 534340a..6cf83e2 100644
--- a/07/solution.c
+++ b/07/solution.c
@@ -5,6 +5,7 @@
 #include <fcntl.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
+#include <ctype.h>
 
 uint32_t fnv1a_32(const char *buf, size_t len)
 {
@@ -80,7 +81,8 @@ int main(int argc, char **argv) {
         } else if (l[0] == 'd') {
             addent(current, l + 4);
         } else {
-            size_t sz = strtoul(l, NULL, 10);
+            size_t sz = 0;
+            while (isdigit(*l)) sz = 10 * sz + (*l++ - '0');
             entry *f = current;
             for (; f->parent; f = f->parent)
                 f->size += sz;