Error: case label.. Not sure what GCC means.

fig04_07.c: In function ‘main’:
fig04_07.c:35:13: error: case label does not reduce to an integer constant
fig04_07.c:38:13: error: case label does not reduce to an integer constant
fig04_07.c:41:13: error: case label does not reduce to an integer constant
fig04_07.c:44:13: error: case label does not reduce to an integer constant
fig04_07.c:47:13: error: case label does not reduce to an integer constant
fig04_07.c:50:13: error: case label does not reduce to an integer constant
fig04_07.c:51:13: error: case label does not reduce to an integer constant

#include <stdio.h>

int main(void){
    int grade;
    int aCount = 0;
    int bCount = 0;
    int cCount = 0;
    int dCount = 0;
    int fCount = 0;

    printf("Enter the letter grades\n");
    printf("Enter the OEF characterto end the input\n");

    while ((grade = getchar()) != EOF){
        if (grade <= 90) { grade -= 32; }
        switch (grade) {
            case "A":
                ++aCount;
                break;
            case "B":
                ++bCount;
                break;
            case "C":
                ++cCount;
                break;
            case "D":
                ++dCount;
                break;
            case "F":
                ++fCount;
                break;
            case "\n":
            case "\t":
            case ' ':
                break;
            default:
                printf("Incorrect letter grade entered!\n");
                printf("Enter a new grade\n");
                break;
        }
    }
    return 0;
}