diff --git a/curve25519-dalek/src/field.rs b/curve25519-dalek/src/field.rs
index 492b05c..78b43ec 100644
--- a/curve25519-dalek/src/field.rs
+++ b/curve25519-dalek/src/field.rs
@@ -28,7 +28,7 @@
 use cfg_if::cfg_if;
 
 use subtle::Choice;
-use subtle::ConditionallyNegatable;
+
 use subtle::ConditionallySelectable;
 use subtle::ConstantTimeEq;
 
@@ -288,7 +288,7 @@ impl FieldElement {
 
         // Choose the nonnegative square root.
         let r_is_negative = r.is_negative();
-        r.conditional_negate(r_is_negative);
+        let r_neg = -&r; r.conditional_assign(&r_neg, r_is_negative);
 
         let was_nonzero_square = correct_sign_sqrt | flipped_sign_sqrt;
 
@@ -314,6 +314,7 @@ impl FieldElement {
 
 #[cfg(test)]
 mod test {
+    use subtle::ConditionallyNegatable;
     use crate::field::*;
 
     /// Random element a of GF(2^255-19), from Sage