diff --git a/include/alisp/sv.h b/include/alisp/sv.h
index aac6777..72f7f07 100644
--- a/include/alisp/sv.h
+++ b/include/alisp/sv.h
@@ -29,8 +29,8 @@ typedef struct
 sv_t sv_copy(sv_t);
 sv_t sv_chop_left(sv_t, u64 size);
 sv_t sv_chop_right(sv_t, u64 size);
-sv_t sv_substr(sv_t, u64 position, u64 size);
 sv_t sv_truncate(sv_t, u64 newsize);
+sv_t sv_substr(sv_t, u64 position, u64 size);
 
 sv_t sv_till(sv_t, const char *reject);
 sv_t sv_while(sv_t, const char *accept);
diff --git a/src/sv.c b/src/sv.c
index e9e8073..d295af7 100644
--- a/src/sv.c
+++ b/src/sv.c
@@ -36,18 +36,19 @@ sv_t sv_chop_right(sv_t sv, u64 size)
   return SV(sv.data, sv.size - size);
 }
 
-sv_t sv_substr(sv_t sv, u64 position, u64 size)
-{
-  return sv_truncate(sv_chop_left(sv, position), size);
-}
-
 sv_t sv_truncate(sv_t sv, u64 newsize)
 {
-  if (newsize >= sv.size)
-    return sv;
+  if (newsize > sv.size)
+    return SV(NULL, 0);
   return SV(sv.data, newsize);
 }
 
+sv_t sv_substr(sv_t sv, u64 position, u64 size)
+{
+  sv_t result = sv_truncate(sv_chop_left(sv, position), size);
+  return result;
+}
+
 sv_t sv_till(sv_t sv, const char *reject)
 {
   if (sv.size == 0 || !sv.data)