diff --git a/lib/fixRanges.spec.ts b/lib/fixRanges.spec.ts
index e5c01c6..81e533d 100644
--- a/lib/fixRanges.spec.ts
+++ b/lib/fixRanges.spec.ts
@@ -73,6 +73,7 @@ describe('fixRanges prefixes', () => {
     isFixed('=[Foo]Ab12!B2', "='[Foo]Ab12'!B2");
     isFixed('=ABC123!B2', "='ABC123'!B2");
     isFixed('=abc123!B2', "='abc123'!B2");
+    isFixed('=2020plan!B2', "='2020plan'!B2");
     isFixed('=C!B2', "='C'!B2");
     isFixed('=R!B2', "='R'!B2");
     isFixed('=RC!B2', "='RC'!B2");
@@ -94,6 +95,7 @@ describe('fixRanges prefixes', () => {
     isFixed('=SUM(B:8!A1)', "=SUM('B:8'!A1)");
     isFixed('=SUM(8:D!A1)', "=SUM('8:D'!A1)");
     isFixed('=SUM(10:23!A1)', "=SUM('10:23'!A1)");
+    isFixed('=SUM(2020plan:Mar!A1)', "=SUM('2020plan:Mar'!A1)");
     isFixed('=A:B!A1', '=A:B!A1');
     isFixed('=SUM(AA:AB!A1)', '=SUM(AA:AB!A1)');
     isFixed('=SUM(A:AB!A1)', '=SUM(A:AB!A1)');
diff --git a/lib/lexers/lexContext.ts b/lib/lexers/lexContext.ts
index 35a074e..3b3d9da 100644
--- a/lib/lexers/lexContext.ts
+++ b/lib/lexers/lexContext.ts
@@ -8,6 +8,24 @@ const EXCL = 33; // !
 const COLON = 58; // :
 const PERIOD = 46; // .
 
+// [0-9A-Za-z._¡¤§¨ª\u00ad¯-\uffff]
+export function isUnquotedSheetNameChar (c: number): boolean {
+  return (
+    (c >= 65 && c <= 90) || // A-Z
+    (c >= 97 && c <= 122) || // a-z
+    (c >= 48 && c <= 57) || // 0-9
+    (c === 46) || // .
+    (c === 95) || // _
+    (c === 161) || // ¡
+    (c === 164) || // ¤
+    (c === 167) || // §
+    (c === 168) || // ¨
+    (c === 170) || // ª
+    (c === 173) || // \u00ad
+    (c >= 175)    // ¯-\uffff
+  );
+}
+
 // xlsx xml uses a variant of the syntax that has external references in
 // bracets. Any of: [1]Sheet1!A1, '[1]Sheet one'!A1, [1]!named
 export function lexContextQuoted (str: string, pos: number, options: { xlsx: boolean }): Token | undefined {
@@ -87,27 +105,9 @@ export function lexContextUnquoted (str: string, pos: number, options: { xlsx: b
         }
         return undefined;
       }
-      else if (
-        (br1 == null || br2 != null) &&
-        // [0-9A-Za-z._¡¤§¨ª\u00ad¯-\uffff]
-        !(
-          (c >= 65 && c <= 90) || // A-Z
-          (c >= 97 && c <= 122) || // a-z
-          (c >= 48 && c <= 57) || // 0-9
-          (c === 46) || // .
-          (c === 95) || // _
-          (c === 161) || // ¡
-          (c === 164) || // ¤
-          (c === 167) || // §
-          (c === 168) || // ¨
-          (c === 170) || // ª
-          (c === 173) || // \u00ad
-          (c >= 175)    // ¯-\uffff
-        )
-      ) {
+      else if ((br1 == null || br2 != null) && !isUnquotedSheetNameChar(c)) {
         return;
       }
-      // 0-9A-Za-z._¡¤§¨ª\u00ad¯-\uffff
       pos++;
     }
   }
diff --git a/lib/lexers/lexNumber.ts b/lib/lexers/lexNumber.ts
index b4490a6..7b9b858 100644
--- a/lib/lexers/lexNumber.ts
+++ b/lib/lexers/lexNumber.ts
@@ -1,5 +1,6 @@
 import { NUMBER } from '../constants.ts';
 import type { Token } from '../types.ts';
+import { isUnquotedSheetNameChar } from './lexContext.ts';
 
 const EXCL = 33; // !
 const COLON = 58; // :
@@ -54,5 +55,19 @@ export function lexNumber (str: string, pos: number): Token | undefined {
     return;
   }
 
+  // A sheet name may begin with digits, so digits that run on into name characters and then
+  // reach a "!" or ":" are the start of a prefix rather than a number: the "2020plan" of
+  // 2020plan!A1, or of Jan:2020plan!A1.
+  if (!frac && isUnquotedSheetNameChar(tail)) {
+    let end = pos;
+    while (end < str.length && isUnquotedSheetNameChar(str.charCodeAt(end))) {
+      end++;
+    }
+    const after = str.charCodeAt(end);
+    if (after === EXCL || after === COLON) {
+      return;
+    }
+  }
+
   return { type: NUMBER, value: str.slice(start, pos) };
 }
diff --git a/lib/tokenize.spec.ts b/lib/tokenize.spec.ts
index bd825c3..a921ddd 100644
--- a/lib/tokenize.spec.ts
+++ b/lib/tokenize.spec.ts
@@ -558,6 +558,36 @@ describe('lexer', () => {
       }
     });
 
+    test('a sheet name that begins with digits is one prefix, not a number and a name', () => {
+      // 2020plan is a sheet name, not the number 2020 and the name plan: Excel stores 2020plan!A1
+      // quoted, as '2020plan'!A1.
+      isTokens('=2020plan!A1', [
+        { type: FX_PREFIX, value: '=' },
+        { type: REF_RANGE, value: '2020plan!A1' }
+      ]);
+      isTokens('=2020plan:Mar!A1', [
+        { type: FX_PREFIX, value: '=' },
+        { type: REF_RANGE, value: '2020plan:Mar!A1' }
+      ]);
+      // as the second name too; which side of the colon it lands on is the range operator's rule
+      expect(tokenize('=SUM(Jan:2020plan!A1)').filter(t => t.type === NUMBER)).toEqual([]);
+      // a number followed by a name, with no ! or : after it, is a number and a name
+      isTokens('=2020plan', [
+        { type: FX_PREFIX, value: '=' },
+        { type: NUMBER, value: '2020' },
+        { type: REF_NAMED, value: 'plan' }
+      ]);
+      isTokens('=UNIQUE(1stLevel!H2:H)', [
+        { type: FX_PREFIX, value: '=' },
+        { type: FUNCTION, value: 'UNIQUE' },
+        { type: OPERATOR, value: '(' },
+        { type: REF_RANGE, value: '1stLevel!H2' },
+        { type: OPERATOR, value: ':' },
+        { type: REF_NAMED, value: 'H' },
+        { type: OPERATOR, value: ')' }
+      ]);
+    });
+
     test('decimals with no fraction part', () => {
       isTokens('=1.*2', [
         { type: FX_PREFIX, value: '=' },