diff --git a/src/ensure.rs b/src/ensure.rs
index fb45baa..4f05a5c 100644
--- a/src/ensure.rs
+++ b/src/ensure.rs
@@ -47,20 +47,24 @@ impl Buf {
 
 impl Write for Buf {
     fn write_str(&mut self, s: &str) -> fmt::Result {
+        if s.bytes().any(|b| b == b' ' || b == b'\n') {
+            return Err(fmt::Error);
+        }
+
         let remaining = self.bytes.len() - self.written;
-        if s.len() <= remaining {
-            unsafe {
-                ptr::copy_nonoverlapping(
-                    s.as_ptr(),
-                    self.bytes.as_mut_ptr().add(self.written).cast::<u8>(),
-                    s.len(),
-                );
-            }
-            self.written += s.len();
-            Ok(())
-        } else {
-            Err(fmt::Error)
+        if s.len() > remaining {
+            return Err(fmt::Error);
+        }
+
+        unsafe {
+            ptr::copy_nonoverlapping(
+                s.as_ptr(),
+                self.bytes.as_mut_ptr().add(self.written).cast::<u8>(),
+                s.len(),
+            );
         }
+        self.written += s.len();
+        Ok(())
     }
 }
 
diff --git a/tests/test_ensure.rs b/tests/test_ensure.rs
index ac69ce9..37d56cb 100644
--- a/tests/test_ensure.rs
+++ b/tests/test_ensure.rs
@@ -366,6 +366,19 @@ fn test_trailer() {
     );
 }
 
+#[test]
+fn test_whitespace() {
+    #[derive(Debug)]
+    pub struct Point {
+        pub x: i32,
+        pub y: i32,
+    }
+
+    let point = Point { x: 0, y: 0 };
+    let test = || Ok(ensure!("" == format!("{:#?}", point)));
+    assert_err(test, "Condition failed: `\"\" == format!(\"{:#?}\", point)`");
+}
+
 #[test]
 fn test_too_long() {
     let test = || Ok(ensure!("" == "x".repeat(10)));