Define this function:

(define atom? (lambda (x) (and (not (pair? x)) (not (null? x)))))

In a scheme buffer write: (atom? ) ^--- SPC

After hitting the space, slime enters sldb:

java.lang.NullPointerException [type java.lang.NullPointerException]

Restarts: 0: [quit] terminate current thread

Backtrace: 0: #<ff gnu/expr/Language.java mangleName 481> Locals: this = #!null reversible = #f kind = -1 name = #!null 1: #<ff gnu/expr/Compilation.java mangleName 751> Locals: this = #!null name = #!null 2: #<ff swank-kawa.scm mangledName 925> Locals: this = #!null f = #<procedure <string>:2> 3: #<ff swank-kawa.scm moduleMethod$GrMethRef 921> Locals: this = #!null f = #<procedure <string>:2> 4: #<ff swank-kawa.scm arglist 847> Locals: this = #!null obj = #<procedure <string>:2> 5: #<ff swank-kawa.scm operatorArglist$V 832> 6: #<ff swank-kawa.scm applyN 828> 7: #<ff gnu/expr/ModuleMethod.java applyN 216> 8: #<ff gnu/kawa/functions/ApplyToArgs.java applyN 139> 9: breakpoint (#<java.lang.NullPointerException>) --more--