diff --git a/src/calc.rs b/src/calc.rs
index 07487c1..a09f7a4 100644
--- a/src/calc.rs
+++ b/src/calc.rs
@@ -260,9 +260,19 @@ impl<'a> Calculator<'a> {
                     ',' => self.display_mode = CalculatorDisplayMode::Default(Some(',')),
                     ' ' => self.display_mode = CalculatorDisplayMode::Default(Some(' ')),
                     's' => self.display_mode = CalculatorDisplayMode::Scientific(3),
-                    'S' => self.display_mode = CalculatorDisplayMode::Scientific(self.pop_usize()?),
+                    'S' => {
+                        let precision = self.checked_get(0)? as usize;
+                        if precision >= 20 {
+                            return Err(CalculatorError::PrecisionTooHigh);
+                        }
+                        self.display_mode = CalculatorDisplayMode::Scientific(self.pop_usize()?)
+                    }
                     'e' => self.display_mode = CalculatorDisplayMode::Engineering(3),
                     'E' => {
+                        let precision = self.checked_get(0)? as usize;
+                        if precision >= 20 {
+                            return Err(CalculatorError::PrecisionTooHigh);
+                        }
                         self.display_mode = CalculatorDisplayMode::Engineering(self.pop_usize()?)
                     }
                     _ => return Err(CalculatorError::NoSuchSetting(c)),
diff --git a/src/calc/errors.rs b/src/calc/errors.rs
index 192204c..2006e7d 100644
--- a/src/calc/errors.rs
+++ b/src/calc/errors.rs
@@ -14,6 +14,7 @@ pub enum CalculatorError {
     NoSuchSetting(char),
     RecursiveMacro(char),
     ParseError,
+    PrecisionTooHigh,
 }
 
 impl fmt::Display for CalculatorError {
@@ -32,6 +33,7 @@ impl fmt::Display for CalculatorError {
             CalculatorError::NoSuchSetting(c) => write!(f, "No such setting '{}'", c),
             CalculatorError::RecursiveMacro(c) => write!(f, "Recursive macro '{}'", c),
             CalculatorError::ParseError => write!(f, "Parse error"),
+            CalculatorError::PrecisionTooHigh => write!(f, "Precision too high"),
         }
     }
 }
diff --git a/src/main.rs b/src/main.rs
index cb8143c..69f1553 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -390,11 +390,12 @@ fn draw_clippy_rect<T: std::io::Write>(c: ClippyRectangle, f: &mut Frame<Termion
 fn fmt_scientific(i: usize, f: f64, precision: usize) -> String {
     let mut ret = format!("{:.precision$E}", f, precision = precision);
     let exp = ret.split_off(ret.find('E').unwrap_or(0));
-    let (pow_sign, exp) = if exp.starts_with("E-") {
-        ('-', &exp[2..])
+    let (pow_sign, exp) = if let Some(stripped) = exp.strip_prefix("E-") {
+        ('-', stripped)
     } else {
         ('+', &exp[1..])
     };
+
     let sign = if !ret.starts_with('-') { " " } else { "" };
     format!(
         "{:>2}: {}{}E{}{:0>pad$}",
@@ -435,7 +436,7 @@ fn fmt_scientific(i: usize, f: f64, precision: usize) -> String {
 fn fmt_separated(i: usize, f: f64, sep: char) -> String {
     let mut ret = f.to_string();
     let start = if ret.starts_with('-') { 1 } else { 0 };
-    let end = ret.find('.').unwrap_or(ret.len());
+    let end = ret.find('.').unwrap_or_else(|| ret.len());
     for i in 0..((end - start - 1).div_euclid(3)) {
         ret.insert(end - (i + 1) * 3, sep);
     }