diff --git a/reader.c b/reader.c
index bc5b4dc..b63909f 100644
--- a/reader.c
+++ b/reader.c
@@ -74,6 +74,7 @@ static value_t read_one_value(reader_state_t *state)
     return read_special(state);
   case '(':
     return read_list(state);
+  case '-':
   case '0' ... '9':
     return read_number(state);
   case '\"':
@@ -218,6 +219,14 @@ static value_t read_list(reader_state_t *state)
 static value_t read_number(reader_state_t *state)
 {
   fixnum_t num = 0;
+  bool negative = false;
+
+  if (state->ch == '-')
+  {
+    negative = true;
+    next_char(state);
+    release_assert(isdigit(state->ch));
+  }
 
   while (isdigit(state->ch))
   {
@@ -227,6 +236,10 @@ static value_t read_number(reader_state_t *state)
     next_char(state);
   }
 
+  if (negative)
+    num = -num;
+
+  release_assert((FIXNUM_MIN <= num) && (num <= FIXNUM_MAX));
   return fixnum_value(num);
 }