diff --git a/router/src/validation.rs b/router/src/validation.rs
index 9adedc5b..d0ea137d 100644
--- a/router/src/validation.rs
+++ b/router/src/validation.rs
@@ -116,12 +116,16 @@ impl Validation {
             // In this case, we don't know the real length in tokens of the inputs
             // However, the inputs will be truncated by the python servers
             // We make sure that truncate + max_new_tokens <= self.max_total_tokens
-            let input_length = truncate.unwrap_or(self.max_input_length);
             let max_new_tokens: u32 = if let Some(max_new_tokens) = max_new_tokens {
                 max_new_tokens
             } else {
-                self.max_total_tokens.saturating_sub(input_length) as u32
+                if let Some(truncate) = truncate {
+                    self.max_total_tokens.saturating_sub(truncate) as u32
+                } else {
+                    return Err(ValidationError::UnsetMaxNewTokens)
+                }
             };
+            let input_length = truncate.unwrap_or(self.max_input_length);
 
             // Validate MaxNewTokens
             if (input_length as u32 + max_new_tokens) > self.max_total_tokens as u32 {
@@ -393,6 +397,8 @@ pub enum ValidationError {
     Truncate(usize, usize),
     #[error("`typical_p` must be > 0.0 and < 1.0")]
     TypicalP,
+    #[error("one of `max_new_tokens` or `truncate` must be set if a fast tokenizer is not in use")]
+    UnsetMaxNewTokens,
     #[error("`max_new_tokens` must be strictly positive")]
     NegativeMaxNewTokens,
     #[error("`max_new_tokens` must be <= {0}. Given: {1}")]