diff --git a/src/national_number.rs b/src/national_number.rs
index bc2dc7c..301a1b0 100644
--- a/src/national_number.rs
+++ b/src/national_number.rs
@@ -22,13 +22,16 @@ pub struct NationalNumber {
 }
 
 impl NationalNumber {
-    pub fn new(value: u64, zeros: u8) -> Self {
+    pub fn new(value: u64, zeros: u8) -> Result<Self, crate::error::Parse> {
         // E.164 specifies a maximum of 15 decimals, which corresponds to slightly over 48.9 bits.
         // 56 bits ought to cut it here.
-        assert!(value < (1 << 56), "number too long");
-        Self {
-            value: ((zeros as u64) << 56) | value,
+        if value >= (1 << 56) {
+            return Err(crate::error::Parse::TooLong);
         }
+
+        Ok(Self {
+            value: ((zeros as u64) << 56) | value,
+        })
     }
 
     /// The number without any leading zeroes.
diff --git a/src/parser/mod.rs b/src/parser/mod.rs
index 0e3110f..c5914a9 100644
--- a/src/parser/mod.rs
+++ b/src/parser/mod.rs
@@ -87,7 +87,7 @@ pub fn parse_with<S: AsRef<str>>(
         national: NationalNumber::new(
             number.national.parse()?,
             number.national.chars().take_while(|&c| c == '0').count() as u8,
-        ),
+        )?,
 
         extension: number.extension.map(|s| Extension(s.into_owned())),
         carrier: number.carrier.map(|s| Carrier(s.into_owned())),
@@ -109,7 +109,7 @@ mod test {
                 source: country::Source::Default,
             },
 
-            national: NationalNumber::new(33316005, 0),
+            national: NationalNumber::new(33316005, 0).unwrap(),
 
             extension: None,
             carrier: None,
@@ -197,7 +197,7 @@ mod test {
                 source: country::Source::Number,
             },
 
-            national: NationalNumber::new(64123456, 0),
+            national: NationalNumber::new(64123456, 0).unwrap(),
 
             extension: None,
             carrier: None,
@@ -215,7 +215,7 @@ mod test {
                     source: country::Source::Default,
                 },
 
-                national: NationalNumber::new(30123456, 0),
+                national: NationalNumber::new(30123456, 0).unwrap(),
 
                 extension: None,
                 carrier: None,
@@ -230,7 +230,7 @@ mod test {
                     source: country::Source::Plus,
                 },
 
-                national: NationalNumber::new(2345, 0,),
+                national: NationalNumber::new(2345, 0,).unwrap(),
 
                 extension: None,
                 carrier: None,
@@ -245,7 +245,7 @@ mod test {
                     source: country::Source::Default,
                 },
 
-                national: NationalNumber::new(12, 0,),
+                national: NationalNumber::new(12, 0,).unwrap(),
 
                 extension: None,
                 carrier: None,
@@ -260,7 +260,7 @@ mod test {
                     source: country::Source::Default,
                 },
 
-                national: NationalNumber::new(3121286979, 0),
+                national: NationalNumber::new(3121286979, 0).unwrap(),
 
                 extension: None,
                 carrier: Some("12".into()),