diff --git a/src/qcbor_decode.c b/src/qcbor_decode.c
index 693152f..332314b 100644
--- a/src/qcbor_decode.c
+++ b/src/qcbor_decode.c
@@ -884,10 +884,8 @@
          pDecodedItem->uDataType = QCBOR_TYPE_INT64;
 
       } else {
-         /* C can't represent a negative integer in this range so it
-          * is an error.
-          */
-         uReturn = QCBOR_ERR_INT_OVERFLOW;
+         pDecodedItem->val.uint64 = uArgument;
+         pDecodedItem->uDataType  = QCBOR_TYPE_65BIT_NEG_INT;
       }
    }
 
@@ -4921,6 +4919,12 @@
          }
          break;
 
+      case QCBOR_TYPE_65BIT_NEG_INT:
+         /* This type occurs if the value won't fit into int64_t
+          * so this is always an error. */
+         return QCBOR_ERR_CONVERSION_UNDER_OVER_FLOW;
+         break;
+
       default:
          return  QCBOR_ERR_UNEXPECTED_TYPE;
    }
@@ -5291,12 +5295,15 @@
 
       case QCBOR_TYPE_UINT64:
          if(uConvertTypes & QCBOR_CONVERT_TYPE_XINT64) {
-            *puValue =  pItem->val.uint64;
+            *puValue = pItem->val.uint64;
          } else {
             return QCBOR_ERR_UNEXPECTED_TYPE;
          }
          break;
 
+      case QCBOR_TYPE_65BIT_NEG_INT:
+         return QCBOR_ERR_NUMBER_SIGN_CONVERSION;
+
       default:
          return QCBOR_ERR_UNEXPECTED_TYPE;
    }
@@ -5634,6 +5641,10 @@
          return QCBOR_ERR_HW_FLOAT_DISABLED;
 #endif /* QCBOR_DISABLE_FLOAT_HW_USE */
 
+      case QCBOR_TYPE_65BIT_NEG_INT:
+         *pdValue = -(double)pItem->val.uint64 - 1;
+         break;
+
       default:
          return QCBOR_ERR_UNEXPECTED_TYPE;
    }
diff --git a/src/qcbor_encode.c b/src/qcbor_encode.c
index 146253d..a2a944a 100644
--- a/src/qcbor_encode.c
+++ b/src/qcbor_encode.c
@@ -665,6 +665,18 @@
 
 
 /*
+ * Public functions for adding negative integers. See qcbor/qcbor_encode.h
+ */
+void QCBOREncode_AddNegativeUInt64(QCBOREncodeContext *pMe, const uint64_t uValue)
+{
+   // TODO: Error out in dCBOR mode
+   QCBOREncode_Private_AppendCBORHead(pMe, CBOR_MAJOR_TYPE_NEGATIVE_INT, uValue, 0);
+
+   QCBOREncode_Private_IncrementMapOrArrayCount(pMe);
+}
+
+
+/*
  * Public functions for adding signed integers. See qcbor/qcbor_encode.h
  */
 void
@@ -676,9 +688,9 @@
    if(nNum < 0) {
       /* In CBOR -1 encodes as 0x00 with major type negative int.
        * First add one as a signed integer because that will not
-       * overflow. Then change the sign as needed for encoding.  (The
+       * overflow. Then change the sign as needed for encoding (the
        * opposite order, changing the sign and subtracting, can cause
-       * an overflow when encoding INT64_MIN. */
+       * an overflow when encoding INT64_MIN). */
       int64_t nTmp = nNum + 1;
       uValue = (uint64_t)-nTmp;
       uMajorType = CBOR_MAJOR_TYPE_NEGATIVE_INT;