diff --git a/src/field.rs b/src/field.rs
index d4cdb72..0d8cc1c 100644
--- a/src/field.rs
+++ b/src/field.rs
@@ -146,11 +146,11 @@ impl<'b> SubAssign<&'b FieldElement> for FieldElement {
     fn sub_assign(&mut self, _rhs: &'b FieldElement) {
         // To avoid underflow, first add p
         // XXX how many copies should we add to preserve headroom?
-        self.0[0] += constants::p.0[0];
-        self.0[1] += constants::p.0[1];
-        self.0[2] += constants::p.0[2];
-        self.0[3] += constants::p.0[3];
-        self.0[4] += constants::p.0[4];
+        self.0[0] += 2*constants::p.0[0];
+        self.0[1] += 2*constants::p.0[1];
+        self.0[2] += 2*constants::p.0[2];
+        self.0[3] += 2*constants::p.0[3];
+        self.0[4] += 2*constants::p.0[4];
         // then subtract _rhs
         self.0[0] -= _rhs.0[0];
         self.0[1] -= _rhs.0[1];