diff --git a/msvc/h/config.h.msvc6 b/msvc/h/config.h.msvc6
index 95c07d412..5e0b6cb83 100644
--- a/msvc/h/config.h.msvc6
+++ b/msvc/h/config.h.msvc6
@@ -167,7 +167,7 @@ typedef unsigned int cl_hashkey;
 /* #undef HAVE_SYS_IOCTL_H */
 /* putenv() or setenv()							*/
 #undef HAVE_SETENV
-#undef HAVE_PUTENV
+#define HAVE_PUTENV 1
 /* times() and sys/times.h						*/
 /* #undef HAVE_TIMES */
 /* user home directory, user name, etc...				*/
diff --git a/src/c/main.d b/src/c/main.d
index 17e8a6693..ae8991ed1 100644
--- a/src/c/main.d
+++ b/src/c/main.d
@@ -496,8 +496,12 @@ si_setenv(cl_object var, cl_object value)
 		 * (si:setenv "foo" nil), then (si:getenv "foo) returns
 		 * the right thing. */
 		unsetenv(var->string.self);
+#else
+#if defined(_MSC_VER) || defined(mingw32)
+		si_setenv(var, make_simple_string(""));
 #else
 		putenv(var->string.self);
+#endif
 #endif
 		ret_val = 0;
 	} else {
@@ -508,6 +512,8 @@ si_setenv(cl_object var, cl_object value)
 		cl_object temp =
 		  cl_format(4, Cnil, make_constant_string("~A=~A"), var,
 			    value);
+		if (temp->string.hasfillp && temp->string.fillp < temp->string.dim)
+		  temp->string.self[temp->string.fillp] = '\0';
 		putenv(temp->string.self);
 #endif
 	}