diff --git a/Doc/library/math.rst b/Doc/library/math.rst
index 0083409..3c41672 100644
--- a/Doc/library/math.rst
+++ b/Doc/library/math.rst
@@ -40,9 +40,6 @@
    *y*.  On platforms that support signed zeros, ``copysign(1.0, -0.0)``
    returns *-1.0*.
 
-   If *x* is NaN, *y* is ignored and NaN is returned.  If *y* is NaN,
-   it is treated as positive: ``copysign(-1.0, NaN)`` returns 1.0.
-
 .. function:: fabs(x)
 
    Return the absolute value of *x*.
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index 00e2612..7f094ff 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -908,9 +908,7 @@
 FUNC2(copysign, copysign,
       "copysign(x, y)\n\nReturn a float with the magnitude (absolute value) "
       "of x but the sign \nof y. On platforms that support signed zeros, "
-      "copysign(1.0, -0.0) \nreturns -1.0.\n\n"
-      "If x is NaN, y is ignored and NaN is returned. If y is NaN, it is\n"
-      "treated as positive.")
+      "copysign(1.0, -0.0) \nreturns -1.0.\n")
 FUNC1(cos, cos, 0,
       "cos(x)\n\nReturn the cosine of x (measured in radians).")
 FUNC1(cosh, cosh, 1,