Enum type Warning in C

I am writing a code on lpc1788 ARM Cortex M3. I came across a strange warning when I tried to configure the ports as GPIO. Despite of the warning, the code works absolutely fine, but to learn why this warning come, I am putting forward this post here. Following is the code that I have written.

static uint32_t * PIN_GetPointer(uint8_t portnum, uint8_t pinnum)
{
uint32_t *pPIN = NULL;
pPIN = (uint32_t *)(LPC_IOCON_BASE + ((portnum * 32 + pinnum)*sizeof(uint32_t)));
return pPIN;
}

void PINSEL_SetPinMode ( uint8_t portnum, uint8_t pinnum, PinSel_BasicMode modenum)
{
uint32_t *pPIN = NULL;
pPIN = PIN_GetPointer(portnum, pinnum);
*(uint32_t *)pPIN &= ~(3<<3); //Clear function bits
*(uint32_t *)pPIN |= (uint32_t)(modenum<<3);
}

int main(void)
{
PINSEL_SetPinMode(1,15,0); //this gave a warning: enumerated type mixed with another type
PINSEL_SetPinMode(1,18,PINSEL_BASICMODE_NPLU_NPDN); //this doesnt give any warning

/* Following is the enum present in a GPIO related header file, putting it here in comments so that
those who are going through this post, can see the enum

typedef enum
{
PINSEL_BASICMODE_NPLU_NPDN = 0, // Neither Pull up nor pull down
PINSEL_BASICMODE_PULLDOWN, // Pull-down enabled
PINSEL_BASICMODE_PULLUP, // Pull-up enabled (default)
PINSEL_BASICMODE_REPEATER // Repeater mode
}PinSel_BasicMode;
*/

return 0;
}

You have brought up the warning by specifying enum type for the formal parameter and supplying an integer literal. Although not all compilers will flag out a warning in this case, it seems reasonable, enforcing usage of enum constants rather than equivocal numbers. If you don't want it, specify a numeric type for modenum.