diff --git a/libsdhcdrivers/include/sdhc/mmc.h b/libsdhcdrivers/include/sdhc/mmc.h
index 83f160b..f72c026 100644
--- a/libsdhcdrivers/include/sdhc/mmc.h
+++ b/libsdhcdrivers/include/sdhc/mmc.h
@@ -91,6 +91,6 @@
 
 /** Get card capacity
  * @param[in] mmc_card  A handle to an initialised MMC card
- * @return              Card capacity in bytes
+ * @return              Card capacity in bytes, negative on failure.
  */
-unsigned long long mmc_card_capacity(mmc_card_t mmc_card);
+long long mmc_card_capacity(mmc_card_t mmc_card);
diff --git a/libsdhcdrivers/src/mmc.c b/libsdhcdrivers/src/mmc.c
index bba9237..98156ba 100644
--- a/libsdhcdrivers/src/mmc.c
+++ b/libsdhcdrivers/src/mmc.c
@@ -532,10 +532,10 @@
                MMC_WRITE_BLOCK);
 }
 
-unsigned long long mmc_card_capacity(mmc_card_t mmc_card)
+long long mmc_card_capacity(mmc_card_t mmc_card)
 {
     int ret;
-    unsigned long long capacity;
+    long long capacity;
     struct csd csd;
 
     ret = mmc_decode_csd(mmc_card, &csd);