diff --git a/spec/compiler/lexer/lexer_spec.cr b/spec/compiler/lexer/lexer_spec.cr
index 6a858baeaae8..612e16512da7 100644
--- a/spec/compiler/lexer/lexer_spec.cr
+++ b/spec/compiler/lexer/lexer_spec.cr
@@ -321,6 +321,9 @@ describe "Lexer" do
   assert_syntax_error "4F64", "unexpected token: F64"
   assert_syntax_error "0F32", "unexpected token: F32"
 
+  assert_syntax_error ".42", ".1 style number literal is not supported, put 0 before dot"
+  assert_syntax_error "-.42", ".1 style number literal is not supported, put 0 before dot"
+
   it "lexes not instance var" do
     lexer = Lexer.new "!@foo"
     token = lexer.next_token
diff --git a/src/compiler/crystal/syntax/lexer.cr b/src/compiler/crystal/syntax/lexer.cr
index 708a58b63e30..fbaf61bdb159 100644
--- a/src/compiler/crystal/syntax/lexer.cr
+++ b/src/compiler/crystal/syntax/lexer.cr
@@ -636,6 +636,8 @@ module Crystal
       when '~'
         next_char :"~"
       when '.'
+        line = @line_number
+        column = @column_number
         case next_char
         when '.'
           case next_char
@@ -644,6 +646,8 @@ module Crystal
           else
             @token.type = :".."
           end
+        when .ascii_number?
+          raise ".1 style number literal is not supported, put 0 before dot", line, column
         else
           @token.type = :"."
         end