diff --git a/Cargo.toml b/Cargo.toml
index c9f71ed..dcff986 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "http-signature-normalization"
 description = "An HTTP Signatures library that leaves the signing to you"
-version = "0.5.0"
+version = "0.5.1"
 authors = ["asonix <asonix@asonix.dog>"]
 license-file = "LICENSE"
 readme = "README.md"
diff --git a/src/lib.rs b/src/lib.rs
index 7dd45d2..2df9869 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -228,14 +228,6 @@ fn build_signing_string(
     btm: &mut BTreeMap<String, String>,
     mut required_headers: HashSet<String>,
 ) -> Result<String, RequiredError> {
-    for key in btm.keys() {
-        required_headers.remove(key);
-    }
-
-    if !required_headers.is_empty() {
-        return Err(RequiredError(required_headers));
-    }
-
     let request_target = format!("{} {}", method.to_string().to_lowercase(), path_and_query);
 
     btm.insert(REQUEST_TARGET.to_owned(), request_target.clone());
@@ -248,10 +240,20 @@ fn build_signing_string(
 
     let signing_string = sig_headers
         .iter()
-        .filter_map(|h| btm.remove(h).map(|v| format!("{}: {}", h, v)))
+        .filter_map(|h| {
+            let opt = btm.remove(h).map(|v| format!("{}: {}", h, v));
+            if opt.is_some() {
+                required_headers.remove(h);
+            }
+            opt
+        })
         .collect::<Vec<_>>()
         .join("\n");
 
+    if !required_headers.is_empty() {
+        return Err(RequiredError(required_headers));
+    }
+
     Ok(signing_string)
 }