diff --git a/exercises/nucleotide-count/example.rs b/exercises/nucleotide-count/example.rs
index 603e1a4bf..e5967218e 100644
--- a/exercises/nucleotide-count/example.rs
+++ b/exercises/nucleotide-count/example.rs
@@ -3,11 +3,15 @@ use std::collections::HashMap;
 static VALID_NUCLEOTIDES: &'static str = "ACGT";
 
 pub fn count(nucleotide: char, input: &str) -> Result<usize, char> {
-    let valid = |x: char| { VALID_NUCLEOTIDES.contains(x) };
-    if valid(nucleotide) && input.chars().all(valid) {
-        Ok(input.chars().filter(|&c| c == nucleotide).count())
-    } else {
+    let valid = |x: char| VALID_NUCLEOTIDES.contains(x);
+
+    if !valid(nucleotide) {
         Err(nucleotide)
+    } else {
+        match input.chars().find(|&c| !valid(c)) {
+            Some(c) => Err(c),
+            None => Ok(input.chars().filter(|&c| c == nucleotide).count()),
+        }
     }
 }
 
diff --git a/exercises/nucleotide-count/src/lib.rs b/exercises/nucleotide-count/src/lib.rs
index 8b1378917..341c5f145 100644
--- a/exercises/nucleotide-count/src/lib.rs
+++ b/exercises/nucleotide-count/src/lib.rs
@@ -1 +1,16 @@
+use std::collections::HashMap;
 
+pub fn count(nucleotide: char, dna: &str) -> Result<usize, char> {
+    unimplemented!(
+        "How much of nucleotide type '{}' is contained inside DNA string '{}'?",
+        nucleotide,
+        dna
+    );
+}
+
+pub fn nucleotide_counts(dna: &str) -> Result<HashMap<char, usize>, char> {
+    unimplemented!(
+        "How much of every nucleotide type is contained inside DNA string '{}'?",
+        dna
+    );
+}
diff --git a/exercises/nucleotide-count/tests/nucleotide-count.rs b/exercises/nucleotide-count/tests/nucleotide-count.rs
index 53f75ac23..d6eb47f76 100644
--- a/exercises/nucleotide-count/tests/nucleotide-count.rs
+++ b/exercises/nucleotide-count/tests/nucleotide-count.rs
@@ -29,13 +29,13 @@ fn test_count_empty() {
 #[test]
 #[ignore]
 fn count_invalid_nucleotide() {
-    assert!(dna::count('X', "A").is_err());
+    assert_eq!(dna::count('X', "A"), Err('X'));
 }
 
 #[test]
 #[ignore]
 fn count_invalid_dna() {
-    assert!(dna::count('A', "AX").is_err());
+    assert_eq!(dna::count('A', "AX"), Err('X'));
 }
 
 #[test]
@@ -81,5 +81,5 @@ fn test_nucleotide_count_counts_all() {
 #[test]
 #[ignore]
 fn counts_invalid_nucleotide_results_in_err() {
-    assert!(dna::nucleotide_counts("GGXXX").is_err());
+    assert_eq!(dna::nucleotide_counts("GGXXX"), Err('X'));
 }