diff --git a/src/main.rs b/src/main.rs
index 9815f65..151353e 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -80,6 +80,10 @@ fn lines_read_noempty<T: BufRead>(lines: &mut Lines<T>) -> Result<Option<String>
 
 fn unfuck_cif_number(number: &str) -> Result<u32> {
     let mut buf = ArrayString::<2048>::new();
+    // the split later will discard leading/trailing whitespace information, so we first check if it's there
+    if number.is_empty() || number.as_bytes()[0] == 32 || number.as_bytes()[number.len()-1] == 32 {
+        return Err(GrammarError(number.to_string()));
+    }
     let split = number.split_whitespace().collect::<ArrayVec<_, 64>>();
     let splitlen = split.len();
     for (i, word) in split.iter().enumerate() {