diff --git a/src/primitives/hrp.rs b/src/primitives/hrp.rs
index b418fc1e..3e3bd497 100644
--- a/src/primitives/hrp.rs
+++ b/src/primitives/hrp.rs
@@ -131,12 +131,12 @@ impl Hrp {
             arr: [u8; MAX_HRP_LEN],
             index: usize,
             error: Option<Error>,
+            has_lower: bool,
+            has_upper: bool,
         }
 
         impl core::fmt::Write for ByteFormatter {
             fn write_str(&mut self, s: &str) -> fmt::Result {
-                let mut has_lower: bool = false;
-                let mut has_upper: bool = false;
                 for ch in s.chars() {
                     let b = ch as u8; // cast ok, `b` unused until `ch` is checked to be ASCII
 
@@ -151,17 +151,17 @@ impl Hrp {
                     }
 
                     if ch.is_ascii_lowercase() {
-                        if has_upper {
+                        if self.has_upper {
                             self.error = Some(MixedCase);
                             break;
                         }
-                        has_lower = true;
+                        self.has_lower = true;
                     } else if ch.is_ascii_uppercase() {
-                        if has_lower {
+                        if self.has_lower {
                             self.error = Some(MixedCase);
                             break;
                         }
-                        has_upper = true;
+                        self.has_upper = true;
                     };
                 }
 
@@ -181,7 +181,13 @@ impl Hrp {
             }
         }
 
-        let mut byte_formatter = ByteFormatter { arr: [0; MAX_HRP_LEN], index: 0, error: None };
+        let mut byte_formatter = ByteFormatter {
+            arr: [0; MAX_HRP_LEN],
+            index: 0,
+            error: None,
+            has_lower: false,
+            has_upper: false,
+        };
 
         write!(byte_formatter, "{}", data).expect("custom Formatter cannot fail");
         if byte_formatter.index == 0 {
@@ -772,4 +778,25 @@ mod tests {
         assert_eq!(hrp.as_bytes()[1], b'X');
         assert_eq!(hrp.as_bytes()[2], b'~');
     }
+
+    #[test]
+    fn test_parse_display_mixed_case_across_boundaries() {
+        let result_lowercase_then_uppercase_static =
+            Hrp::parse_display(format_args!("{}{}", "abc", "DEF"));
+        assert_eq!(
+            result_lowercase_then_uppercase_static.unwrap_err(),
+            Error::MixedCase,
+            "Failed to detect mixed case when lowercase precedes uppercase across boundaries"
+        );
+
+        let lower_abc = "abc".to_string();
+        let upper_def = "DEF".to_string();
+        let result_lowercase_then_uppercase_heap =
+            Hrp::parse_display(format_args!("{}{}", lower_abc, upper_def));
+        assert_eq!(
+            result_lowercase_then_uppercase_heap.unwrap_err(),
+            Error::MixedCase,
+            "Failed to detect mixed case when lowercase precedes uppercase across boundaries"
+        );
+    }
 }