diff --git a/ClosedXML/Excel/Cells/XLCell.cs b/ClosedXML/Excel/Cells/XLCell.cs
index 911910b..799b137 100644
--- a/ClosedXML/Excel/Cells/XLCell.cs
+++ b/ClosedXML/Excel/Cells/XLCell.cs
@@ -382,7 +382,13 @@
                         w => String.Compare(w.Name, sName, true) == 0)
                         && XLHelper.IsValidA1Address(cAddress)
                         )
-                        return _worksheet.Workbook.Worksheet(sName).Cell(cAddress).Value;
+                    {
+                        var referenceCell = _worksheet.Workbook.Worksheet(sName).Cell(cAddress);
+                        if (referenceCell.IsEmpty(false))
+                            return 0;
+                        else
+                            return referenceCell.Value;
+                    }
 
                     var retVal = Worksheet.Evaluate(fA1);
                     var retValEnumerable = retVal as IEnumerable;
@@ -2194,7 +2200,7 @@
                 }
                 Worksheet.EventTrackingEnabled = eventTracking;
             }
-            
+
             return this;
         }
 
diff --git a/ClosedXML_Tests/Excel/Cells/XLCellTests.cs b/ClosedXML_Tests/Excel/Cells/XLCellTests.cs
index 007b492..4f1180d 100644
--- a/ClosedXML_Tests/Excel/Cells/XLCellTests.cs
+++ b/ClosedXML_Tests/Excel/Cells/XLCellTests.cs
@@ -510,5 +510,41 @@
                 Assert.AreEqual("E8:J13", ws.Cell("J13").CurrentRegion.RangeAddress.ToString());
             }
         }
+
+        // https://github.com/ClosedXML/ClosedXML/issues/630
+        [Test]
+        public void ConsiderEmptyValueAsNumericInSumFormula()
+        {
+            using (var wb = new XLWorkbook())
+            {
+                var ws = wb.AddWorksheet("Sheet1");
+
+                ws.Cell("A1").SetValue("Empty");
+                ws.Cell("A2").SetValue("Numeric");
+                ws.Cell("A3").SetValue("Copy of numeric");
+
+                ws.Cell("B2").SetFormulaA1("=B1");
+                ws.Cell("B3").SetFormulaA1("=B2");
+
+                ws.Cell("C2").SetFormulaA1("=SUM(C1)");
+                ws.Cell("C3").SetFormulaA1("=C2");
+
+                object b1 = ws.Cell("B1").Value;
+                object b2 = ws.Cell("B2").Value;
+                object b3 = ws.Cell("B3").Value;
+
+                Assert.AreEqual("", b1);
+                Assert.AreEqual(0, b2);
+                Assert.AreEqual(0, b3);
+
+                object c1 = ws.Cell("C1").Value;
+                object c2 = ws.Cell("C2").Value;
+                object c3 = ws.Cell("C3").Value;
+
+                Assert.AreEqual("", c1);
+                Assert.AreEqual(0, c2);
+                Assert.AreEqual(0, c3);
+            }
+        }
     }
 }