diff --git a/py/formatfloat.c b/py/formatfloat.c
index 22dd8aaacc..60dcee6f54 100644
--- a/py/formatfloat.c
+++ b/py/formatfloat.c
@@ -258,7 +258,7 @@ int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, ch
         }
 
         // It can be that f was right on the edge of an entry in pos_pow needs to be reduced
-        if (f >= FPCONST(10.0)) {
+        if ((int)f >= 10) {
             e += 1;
             f *= FPCONST(0.1);
         }
diff --git a/tests/float/float_format.py b/tests/float/float_format.py
index 4d5ad1d693..cda395ce02 100644
--- a/tests/float/float_format.py
+++ b/tests/float/float_format.py
@@ -9,3 +9,7 @@ for val in (116, 1111, 1234, 5010, 11111):
 # make sure rounding is done at the correct precision
 for prec in range(8):
     print(('%%.%df' % prec) % 6e-5)
+
+# check certain cases that had a digit value of 10 render as a ":" character
+print('%.2e' % float('9' * 51 + 'e-39'))
+print('%.2e' % float('9' * 40 + 'e-21'))