diff --git a/builtins-test/tests/addsub.rs b/builtins-test/tests/addsub.rs
index 859de3e42..65bafec37 100644
--- a/builtins-test/tests/addsub.rs
+++ b/builtins-test/tests/addsub.rs
@@ -108,6 +108,25 @@ macro_rules! float_sum {
                         );
                     }
                 });
+
+                // Quiet sNaN by setting the top significand bit. `sub(x, nan)`
+                // flips that sign because sub is add with b's sign bit inverted.
+                let one = <$f as Float>::ONE;
+                for input in [
+                    <$f as Float>::SNAN,
+                    <$f as Float>::NAN,
+                    <$f as Float>::NEG_SNAN,
+                    <$f as Float>::NEG_NAN,
+                ] {
+                    let expected =
+                        <$f>::from_bits(input.to_bits() | <$f as Float>::SIG_TOP_BIT);
+                    assert_eq!($fn_add(input, one).to_bits(), expected.to_bits());
+                    assert_eq!($fn_add(one, input).to_bits(), expected.to_bits());
+                    assert_eq!($fn_sub(input, one).to_bits(), expected.to_bits());
+                    let flipped =
+                        <$f>::from_bits(expected.to_bits() ^ <$f as Float>::SIGN_MASK);
+                    assert_eq!($fn_sub(one, input).to_bits(), flipped.to_bits());
+                }
             }
         )*
     }
diff --git a/compiler-builtins/src/float/add.rs b/compiler-builtins/src/float/add.rs
index 6503bc37d..6481610d9 100644
--- a/compiler-builtins/src/float/add.rs
+++ b/compiler-builtins/src/float/add.rs
@@ -33,11 +33,11 @@ where
     if a_abs.wrapping_sub(one) >= inf_rep - one || b_abs.wrapping_sub(one) >= inf_rep - one {
         // NaN + anything = qNaN
         if a_abs > inf_rep {
-            return F::from_bits(a_abs | quiet_bit);
+            return F::from_bits(a_rep | quiet_bit);
         }
         // anything + NaN = qNaN
         if b_abs > inf_rep {
-            return F::from_bits(b_abs | quiet_bit);
+            return F::from_bits(b_rep | quiet_bit);
         }
 
         if a_abs == inf_rep {