diff --git a/CHANGELOG.md b/CHANGELOG.md
index 16d5dc9a..bce04eef 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ Features
 
 Bug Fixes
 
+- Fixed a bug in the oprimized numeric comparison operators when comparing a double with a bignum, that could lead to undefined behavior.
 - Fixed `make_empty_bytevector` and `make_c_opaque` parameters on `(cyclone foreign)`.
 
 ## 0.18 - June 8, 2020
diff --git a/runtime.c b/runtime.c
index be64d352..00c379b7 100644
--- a/runtime.c
+++ b/runtime.c
@@ -1818,7 +1818,7 @@ object FUNC_FAST_OP(void *data, object x, object y) { \
     } else if (tx == -1         && ty == bignum_tag) { \
       return Cyc_bignum_cmp(BN_CMP, x, tx, y, ty) ? boolean_t : boolean_f; \
     } else if (tx == double_tag && ty == bignum_tag) { \
-      return (double_value(x)) OP mp_get_double(&bignum_value(x)) ? boolean_t : boolean_f; \
+      return (double_value(x)) OP mp_get_double(&bignum_value(y)) ? boolean_t : boolean_f; \
     } else if (tx == complex_num_tag && ty == complex_num_tag) { \
       return ((complex_num_value(x)) == (complex_num_value(y))) ? boolean_t : boolean_f; \
     } else if (tx == complex_num_tag && ty != complex_num_tag) { \