diff --git a/include/mcl/fp.hpp b/include/mcl/fp.hpp
index b5824ce..c0d37b5 100644
--- a/include/mcl/fp.hpp
+++ b/include/mcl/fp.hpp
@@ -425,7 +425,7 @@ public:
 	}
 	static inline void invG(Unit *y, const Unit *x)
 	{
-		op_.invG(y, x, op_.p);
+		op_.invG(y, x, op_);
 	}
 private:
 	static inline void inFromStr(mpz_class& x, bool *isMinus, const std::string& str, int base)
diff --git a/include/mcl/fp_base.hpp b/include/mcl/fp_base.hpp
index fa6f6e6..9f986a4 100644
--- a/include/mcl/fp_base.hpp
+++ b/include/mcl/fp_base.hpp
@@ -35,8 +35,11 @@ typedef uint32_t Unit;
 typedef uint64_t Unit;
 #endif
 
+struct Op;
+
 typedef void (*void1op)(Unit*);
 typedef void (*void2op)(Unit*, const Unit*);
+typedef void (*void2opOp)(Unit*, const Unit*, const Op&);
 typedef void (*void3op)(Unit*, const Unit*, const Unit*);
 typedef void (*void4op)(Unit*, const Unit*, const Unit*, const Unit*);
 typedef int (*int2op)(Unit*, const Unit*);
@@ -159,7 +162,7 @@ struct Op {
 	void2op fromMont;
 	// require p
 	void3op negG;
-	void3op invG;
+	void2opOp invG;
 	void4op addG;
 	void4op subG;
 	void3op mulPreG;
diff --git a/src/fp.cpp b/src/fp.cpp
index 77b988d..01f903a 100644
--- a/src/fp.cpp
+++ b/src/fp.cpp
@@ -122,12 +122,12 @@ struct OpeFunc {
 		mpz_mod(my, mx, mp);
 		local::clearArray(y, my->_mp_size, N);
 	}
-	static inline void invC(Unit *y, const Unit *x, const Unit *p)
+	static inline void invC(Unit *y, const Unit *x, const Op& op)
 	{
 		mpz_class my;
 		mpz_t mx, mp;
 		set_mpz_t(mx, x);
-		set_mpz_t(mp, p);
+		set_mpz_t(mp, op.p);
 		mpz_invert(my.get_mpz_t(), mx, mp);
 		local::toArray(y, N, my.get_mpz_t());
 	}