diff --git a/src/lpython/parser/tokenizer.re b/src/lpython/parser/tokenizer.re
index 83882cbf40..e9f13fc6f5 100644
--- a/src/lpython/parser/tokenizer.re
+++ b/src/lpython/parser/tokenizer.re
@@ -321,18 +321,25 @@ int Tokenizer::lex(Allocator &al, YYSTYPE &yylval, Location &loc, diag::Diagnost
                 }
                 if (indent) {
                     indent = false;
-                    if (last_indent_length == 0) {
-                        last_indent_type = tok[0];
-                    }
-                    if (last_indent_type == tok[0]) {
-                        indent_length.push_back(cur-tok);
-                        last_indent_length = cur-tok;
-                        RET(TK_INDENT);
+                    if (cur[0] != ' ' && cur[0] != '\t'
+                        && last_indent_length < cur-tok) {
+                        if (last_indent_length == 0) {
+                            last_indent_type = tok[0];
+                        }
+                        if (last_indent_type == tok[0]) {
+                            indent_length.push_back(cur-tok);
+                            last_indent_length = cur-tok;
+                            RET(TK_INDENT);
+                        } else {
+                            token_loc(loc);
+                            throw parser_local::TokenizerError(
+                            "Indentation should be of the same type "
+                            "(either tabs or spaces)", {loc});
+                        }
                     } else {
-                        loc.first++; loc.last++;
+                        token_loc(loc);
                         throw parser_local::TokenizerError(
-                        "Indentation should be of the same type "
-                        "(either tabs or spaces)", {loc});
+                        "Expected an indented block.", {loc});
                     }
                 } else {
                     if(last_token == yytokentype::TK_NEWLINE
@@ -347,7 +354,7 @@ int Tokenizer::lex(Allocator &al, YYSTYPE &yylval, Location &loc, diag::Diagnost
                                 RET(TK_DEDENT);
                             }
                         } else {
-                            loc.first++; loc.last++;
+                            token_loc(loc);
                             throw parser_local::TokenizerError(
                             "Indentation should be of the same type "
                             "(either tabs or spaces)", {loc});